According to documents I have from a PCB vendor, a typical spec for wire-bondable electrolytic soft gold is and 1.97 \$\mu\$in (min) gold over 188 \$\mu\$in (min) nickel. These are called out as IPC minimum values, though I don't have the IPC documents in front of me.
ENEPIG (electroless nickel, electroless palladium, immersion gold) plating can also produce a wire-bondable surface. A typical spec for that is 197 \$\mu\$in nickel, 12 \$\mu\$in palladium and 1.1 \$\mu\$in gold. Again these are all minimum values.
Recently I had success with a design that spec'ed the gold thickness as 25-30 \$\mu\$in, but really you don't want a very thick gold layer because excessive gold does bad things to solder joint reliability.
My response is going to be the opposite to that of @NickAleeev.
I had similar questions in the past about trace current limits and here are my findings. I haven't done real world tests to confirm this (yet).
The standard for current capacity in a trace was established in a document (IPC-2221) which is what alot of online calculators use. The document is old and outdated. The new standard is the IPC-2152. The IPC-2221 is conservative, and if you can go that route it might be best too. If however, you are limited in space, then IPC-2152 would give you better results. Different calculators will give you different answers, depending on what standard they are basing it off. I have only found two calculators that use IPC-2152 and when I asked my fab house, they said that they can't tell me.
Also internal layers while they are sandwiched (in FR4) have a greater thermal conductivity than air. (The link below goes a bit more into it). They will dissipate the heat more to the surface, and if you have a solid plane between your internal layer and the outer layer, you have a pretty good sink for all that heat.
Have a look at a question I had asked in the past [what is the current limit through a trace? ] you can probably just jump to the end of the question and read the answer.
Some advice I got from this, was just build a dummy board, inject your current into a trace, and see how it reacts. All the, this table says this, that table says that, can't beat an experiment that you can test yourself.
Best Answer
Both inner and outer layers require gold as an etchant mask. The outer layers require nickel because the etchant chemistry is different. Difference of chemistry for inner and outer layers is caused by involvement of electroless step for holes walls plating. Typically, palladium is the starter for walls. I read this on the EPA site.
The dominating requirement after all is environmental impact of etchants.