Welp, you've hit a fairly expensive area of MB... AC Compressors. The cheapest way is to do it yourself if you're qualified. An AC compressor should cost around 300-450 dollars plus the reciever/drier which should be replaced anytime the system is opened. While you're at it, convert the r12(freon) to 134A(SUVA). Most places sell a conversion kit including adaptors,oil,hose and valve for about $35 and 134A is much cheaper than r12 (4.99 vs 40.00 per can and you dont need a license for 134a like you do for r12).
PS: If you do it yourself, buy the parts from the Online Part Shop, they're pretty cheap and they are nice enough to let us use their forum.