Disclosure: I currently work for one of the manufacturers mentioned, I completed an internship with a second, and I know current and former employees of a third. I can't reveal specifics but I can give some general reasons why ICs have variable costs and prices. I also can't speak about the specific ICs mentioned -- even if I knew why my company's version is priced the way it is and could reveal that information, I couldn't possibly know why other companies priced theirs differently.

There are many reasons -- both technical and non-technical -- why the price of one manufacturer's IC may be significantly higher than another's. Below are some of the major ones. Some or all of these may be true for a particular case, and manufacturers may be in different price positions for different IC types (e.g. op amps, ADCs, voltage regulators, etc.).

Technical Reasons

Fab/Assembly

Every manufacturer has a different fab process (actually many processes), and the process used by a manufacturer may have better performance for a particular application or the process may be more expensive (which of course drives up the cost and price of the final IC).

Even with identical processes and circuitry, though, the materials used in the assembly of the IC can vary in performance and cost. For example, a higher quality mold compound reduces stress on the die and therefore improves performance over temperature...at an increased manufacturing cost. To further reduce stress on the die a polyimide layer may be added. Another material choice that can affect performance and cost is the wire bond material -- for example, it is easier to meet or exceed qualification standards (e.g. temp cycle) with gold wire but gold is more expensive than copper. The added cost of higher quality materials may be important for applications which require long lifetimes, severe temperature swings, etc., but would be an unnecessary expense for shorter term applications with little temperature variation.

Test

Production test also has a large effect on overall cost and quality. Virtually every IC requires some sort of trim (e.g. laser trimming) for at least an internal bandgap voltage reference or oscillator, and possibly for offset reduction, gain correction, etc. Adding additional trims and/or trim bits can improve the performance of the trimmed IC at the cost of increased test time (which is increased test cost). Trim may also require the addition of non-volatile memory, which may require additional data retention tests that also increase the test time. The fab process may even dictate whether trimming is done at wafer probe or final test (i.e. after the die is packaged); wafer probe generally has higher throughput (so it's cheaper) and allows the manufacturer to throw out bad die before spending money packaging it, which of course reduces overall cost of test.

Also, while every IC is tested (at the very least for continuity) overall test coverage can vary. Some applications like defense, automotive, or medical require very low or 0 DPPM, which requires the manufacturer to fully test all electrical parameters (possibly over temperature, which significantly increases test cost). Other applications do not require such low DPPM and the manufacturer may choose not to test in production certain electrical parameters which demonstrated a high \$C_{pk}\$ during characterization, especially if those parameters have a long test time or require more expensive tester equipment. Skipping these tests can result in a significant cost and price reduction with very low but non-zero risk (due to the high \$C_{pk}\$) of passing a die that does not meet the spec, which may be worth it to customers in less critical applications.

Non-technical Reasons

One non-technical factor affecting price is which manufacturer was the first to market. This manufacturer has a temporary monopoly or near monopoly and can command a higher price. This manufacturer may spend less time optimizing their production for lower cost in order to be first to the market. Manufacturers which enter the market later tend to optimize for cost to undercut the manufacturer that was first to market since a customer will not switch to a different manufacturer for an identical or nearly identical IC at the same price. The manufacturer who was first to market may still be able to command a higher price if they have established design wins with large customers who do not wish to qualify a new manufacturer's IC even if the new IC is offered at a lower price.

Also, a manufacturer's prior relationships with major customers and perceived reputation can allow it to charge a higher price. Major customers may be willing to pay extra if they have an established relationship with a manufacturer's support teams and/or if the customer(s) have had quality problems with a different manufacturer in the past.

In short

Ultimately, a manufacturer's price depends on which market it is targeting: some customers are relatively low volume but have very high quality needs and are willing to pay for it (e.g. military, automotive, and medical) whereas other customers have much higher volumes and every penny counts. ICs manufactured for critical applications depend on higher margins to make up for relatively low volume, use better quality materials, have more extensive test coverage, etc. ICs manufactured for less critical but higher volume applications optimize cost to deliver lower priced ICs which make up for the lower margins with much higher volumes.

Do you have $20? You can solder them for $20. Or for free, if you already have a heat gun.

Why do you think you'll over heat the package? I solder large power transistors, including TO-247 size power transistors and FETs to much more substantial copper than anything in your photo. Like a solid copper heatsink that weighs several pounds. I've yet to have any device be damaged in any measurable way by this. These packages are designed to endure reflow, which usually involves the entire package reaching temperatures of at least 260°C. The bigger TO power packages are designed to withstand several reflow cycles of 260°C for 2 minutes straight, then allowed to cool via natural convection over as long as a 20 minute stretch.

Just use a heatgun/hot air gun like the kind you'd use for heat shrink. 1200W is plenty. Personally, I have one of these. Hardly high end or precision but its effectively a hair dryer with a much lower CFM airflow rate.

Use it to heat the entire copper bus bar to temperature, put a generous amount of solder paste on the back of the transistor, use some pliers and hold it in position against the bus bar. If you heated up the bar enough and it has enough thermal mass, it will bring the transistor up to temperature, melt the paste and flux, and form a very good joint. Keep the transistor immobilized until the solder has completely solidified (which might be a bit below when it stops being a liquid if it is not eutectic - though if you are able, use eutectic solder for this).

That is how you get the highest quality joint, but you can achieve something nearly as good by just slathering on some flux and using wire solder and pliers.

Bringing that much copper up to temperature might seem a little excessive, but that's how its done. You are right, the copper will suck the heat away too fast. Use that to your advantage. Not only can you do the initial joining this way, but it it is perfectly safe and suitable for rework as well.

Entirely due to that great thermal conductivity. That conductivity means that it will spread out heat and become a uniform temperature very quickly, so you can blow air on a somewhat distant section (distant from the semiconductor parts etc.) and this will diffuse the heat and bring everything up to temperature without causing spot overheating. Also, a 1200W heat gun will have no trouble heating those busbars. I can personally attest that it will heat those bus bars up in short order. I've used my 1200W heat gun to heat up 3 pound passive Xeon heatsinks that are solid copper and larger than the aluminum heatsink in your photo, and soldered power components directly to it. They were not damaged from this.

The primary concern for most semiconductors in regards to heat is their epoxy encapsulation, which is what supports the lead frame and bond wires. However, as long as you do not mechanically stress and seriously overheat the epoxy, it is fine. The silicon itself is, well, silicon. Do you know what 260°C does to silicon?

Nothing. It's silicon. It melts at 1414°C. They are heated to well above soldering temperatures post-lithography during manufacture. On the order of 1000°C, often to diffuse dopants into the silicon. They are cooled extremely slowly, so spend no brief interval at such temperatures.

Those are integrated circuit dies. You are just wanting to solder some big beefy discrete power transistors. They are beasts, they're hardy, they are mechanically and thermally very robust. In the case of MOSFETs, the weakest point is some discharge punching a hole through their metal oxide layer, but certainly nothing thermal or mechanical. They most all won't care about some brief heat.

But even sensitive silicon doesn't really care that much. Like CPUs for example. Here is a CPU, an old AMD X64 CPU that hat a maximum operating temperature of 72°C, that I soldered directly to a copper heatsink. With lead-free solder, SAC305. The higher temp stuff that everyone recognizes as simply 'that solder that they hate'.

It is fine and continues service as an OpenBSD routing box that is handling the very traffic that I am using to post this.

I certainly don't recommend such activities, but I hope it proves my point: its ok to get most semiconductors hot. Just watch the epoxy, if there is any.

A note on the oxide build up: In that picture, I actually got the heatsink a bit too hot and excessive surface oxidation of the unfluxed parts occurred. A few hours soak in some water mixed with food grade citric acid from the health food store took care of that. (Only treat copper with this though, not tin or solder or other metals).

Now, if you still have good reasons that prevent you from bringing the bus bar up to temperature, then just solder the parts using Bi58Sn42 solder. It is readily available in paste form, melts at 138°C, is eutectic, and forms a strong bond. It is what is used to solder heat pipes to copper fins in many CPU and GPU heatsinks. Heat pipes contain, at least in this application, usually methanol, so overheating them will build pressure and cause them to burst. Given that heat pipes are basically magic and have thermal conductivities as high as 2 orders of magnitude more than any known material (the heat is moved by the methanol, via heat of vaporization, then condenses at the other end of the heat pipe, allowing longer heat pipes to have 250 fold the thermal conductivity of copper), bringing the entire thing up to temperature is even more so the only option. And it works fine as long as low temperature solder is used. If your parts are going to get hotter than 138°C during operation, there are a huge range of different solder alloys with varying liquidus temperatures.

Soldering is definitely the method you should use though. It will have the best thermal and electrical performance and by a large margin over other mounting options. It's easier. It's cheap. You've done it before. Don't over think this - heat is the least dangerous thing that those transistors will potentially be subjected to in a SSTC build. Trust me. Anyway, good luck!