i found a converted road bike in okay shape for sale with no brakes for 70 dollars at a used shop. that was in 2006. i honestly don't think you'll be able to find a bike that's in rideable condition for less than 100, and you won't be able to make it fixed gear without a new hub at least. you could make it singlespeed, but by then you're just spending money to spend money.

how old are you and what do you do with your time? 250 isn't that much money nowadays in philly. or suburbs. if you're super young, prove to your parents that you care about bicycling and try to get funding for a kilo tt. it's 300 and a much better bike than anything you'll find on craigslist for less than 250.