I got a used HP projector of eBay for S&G's. I think it was around $150. It's certainly nothing great, but it works fine for watching football games in my garage. You can certainly fine something used off ebay, but wouldn't spend more than the bulb might be worth. You kind of take the seller at their word on the bulb life, but you can find some low hour pj's.
I would look for something with a native 720 resolution. Most of the cheaper pjs you will find will be 800x600. They will look ok, but you may be disappointed comparing it to a flat panel TV
Is it solipsistic in here, or is it just me?