Well I can't really tell you much about the AC repair that was $2500. You did not say how much was labor and what parts where used.
Many times on a JD sound guard cab if you have to replace all the cab lines, the compressor, the re-dryer, and the expansion valve the parts can easily be over $1500. Replacing those cab lines is a real fun job too.
So your bill could be outrageous or it may not be.
As for what you need to repair the systems. A good reclaimer and knowledge of how each system works. It is much more than just throwing freon at the system and calling it good. There are several good repair that cover doing AC repairs.
Had a local repair guy that used to not charge very much for "fixing" AC systems. Found out he was just putting in "stop leak" and recharging them for guys. It was all fun and games until the systems started failing and the guy did not know how to fix them correctly. The guys found out that to fix a system after it has the "stop leak" in it is expensive. The only good way to do it is to replace just about the whole system because you can't get the stuff out of the system. When it gets old it flakes off the inside of the lines and plugs the expansion valve.