That is dirt cheap. The mechanic has an interest in your vehicle as he
repaired it previously,and wants to get it right for you.BD has the best
price on a sanden 508/709.About $300 for a new one.This guy wants to get a
new mount ,the compressor, and about 3 labor for what? $650.Maybe you will
need a new reciever/dryer too($35).Negotioate a new one.For this labor he
will install the mount and compressor, and remove your freon(store it) and
reinstall the freon, and add some new freon for sure.He may have to flush
out all the hoses to get the  old comperssor guts out.Typical compressor
replacement is about $1500.Sometimes $1800 or more.Here is the catch.A
remanufactured compressor contains these by definiton (almost):New
clutch/pulley;new shaft seal;new rear valve plate;new oil;new paint.The
inside pistons and bearings and cylinders,ect are not touched in any
way.There are 5 or 7 pistons.The good news is that compressors are made to
not turn on when the freon gets low, so these components are preserved.The
bad news is that when people add freon to the system to get it to turn back
on, they dont add oil . Oil moves around with the freon,so when the freon
leaks out,so does the oil.Do ya feel lucky?Get a rebuilt.Other wise go with
a new one.

As an alternative,you can save about $250 by buying a junkyard Subie
compressor($40).There are many pitfalls here too.A compressor that has been
unused for a year will leak freon.Pass it up!Get a car recently placed in
the junkyard.In the desert compressors wear out frequently, and need
replacing.If a car is newley in the junkyard with a clean ,obviously new
compressor,grab it($40)!The installation labor will be the same without the
new mounting and wiring changout.