Heck, if you want a cheap compressor for airbrushing I couldn't think of anything better then my old standby, the medical compressor. I used a boxy one for two years before I bought my first silent compressor. The little box medical compressor put out about 15 lbs of pressure, fine for AB work. I see them at Ebaby all the time and they go for less than $20. Mine was very much like this one:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2969437288&category=11779
Here are some more little compressors:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2578780063&category=26236
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2364972149&category=22662
There are two more powerful medical compressors at Ebaby right now. They look very much like the more compressors that airbrush users have. The sellers have a starting bid of $1 and $9.99.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2973288070&category=45207
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2581871146&category=11819
You can't beat the price if you are on a budget.