If the compressor is shot, I'd be really surprised if you could get it replaced for $500. That would be one heck of a deal.
When I had to change the A/C compressor on our Workhorse rig several years back, I think the compressor by itself was a bit over $200 from a local auto parts store. I think the Workhorse dealer wanted over $700 for it!!!!
Do you know that the compressor is bad, or could it be just the clutch? If it's just the clutch, that's can be quite a bit cheaper because the system doesn't have to be opened.
We could probably give better advice if you can tell us more of the symptoms you observed. How do you know it's the compressor? If the refrigerant leaks down below a certain level, the compressor won't engage to protect itself. So a leak can mask itself as a bad compressor.
Good Luck,
~Rick