How many BTU cooling is the unit? If it is under 50,000 btu you would be
getting ripped off. HVAC compressors are cheaper than refrigeration. My guess
is about $500 for the compressor and maybe 4 hours of labor at $65/hr so $800
is a fair price.
Ask them what they did to see if compressor is out? Get a second bid from
another company too. Normally to test a compressor they run a test from run and
start or common to the casing to see if it is shorted out. if not than the only
other way it is bad is if the internal valave is friend.