Most urban areas will have something like a bike co-op. See if you can find something with 2 wheels that goes forward.
Seriously though, I picked up my mountain bike, an older Specialized hardrock pro, for $100 on craigslist. All it really needed was some fresh oil on the chain and a new set of tires. I have since bought a commuter specific bike, but the Spec did fine until I could afford something nicer.