Take a look at Craiglist and see what you can find. In my area I've seen several older commercial grade, continuous duty compressors for well under $400.
Yup, I got a 2 cylinder Quincy off craigslist a couple of months ago, it was a steal. Its been working great, although I did end up replacing all the ball valves on the tank because they leaked. It is so much quieter than what I had before. The guy even threw in an extra head and oil pump, although I haven't needed either.
