Cisco 1142 AP not joining WLC

In the "show interface" output, you'll see cumulative counters since the last counter reset ("clear counters []", no specified interface clears counters for all interfaces). This allows you to take snapshots at whatever interval you want. It's the same counters as used by the SNMP management interface (Cacti, MRTG and other monitoring software) and should be relatively easy to parse.

If you're happy scraping the CLI output and do it yourself, that MAY be a quicker way to get up and running with this specific task. I do suspect that some training material and time to install MRTG or Cacti will be worth your time investment, though.

Coverage is the first question. There's nothing magical about the antennas in the higher end APs. They still function similar to the consumer devices with two major exceptions. The first exception is that you can always attach external antennas to boost your signal. The second exception is that you can mess with the antenna power and crank it up and down as necessary. So hopefully that answers your first question.

As for your other question, there's a few reasons to buy the more expensive AP. The major reason is reliability. How often have you had to reboot or replace cheap Linksys APs? The cost difference often isn't worth the labor you throw at those things.

Another reason to spend the money is ease of management. How do you like changing things on each AP individually? Or reprogramming APs individually when you add them or replace them? It's not a great experience. That $10K controller takes that headache away from you. Also, those APs have some nice SNMP features for monitoring, reporting, and extra management with enterprise tools.

The final reason to spend the money on the enterprise APs is for features. Do you want to know if you have Rouge APs? Do you want clients to be able to roam across APs while staying on the same VLAN? How about having the APs automatically adjust their own gain to adapt to current usage and environment? These are the types of things that typically can only be done with enterprise grade wireless APs and controllers. The Linksys gear is never going to do this for you.

Now I don't know your business. I don't know the site. Only you can look at that and determine if it's worth it to invest in more expensive equipment.