Like steevo says pay attention to amps, 5hp peak is different than 5hp running. I picked up a compressor that needed a motor (5 hp running @ ~21 amps) those jokers are expensive!! I didn't think about it when i bought it!
The peak ones are around 3hp running. I called everywhere and there were no where under 200 bucks, iirc weg, emerson,etc had some that wasn't too bad ..i can't recall exact price but i want to say around 300 or so. In the end i found a new one under 200 (including shipping) on ebay (baldor motor) and that required some looking! I'd start there if you're trying for under 200 and need ~21 amp ..maybe something will popup
now the 3hp running ones...those can be found about anywhere like surplus center, etc others have posted.