I see 10 to 20 year old hardtail mountain bikes; Trek 800/820, Specialized Hardrock, GT, Gary Fisher, etc...selling for between $60 and $200 locally on Craigslist. In good condition, something like this should be fine for "tooling around." Alternately, ask at LBS. Maybe they have a trade in that meets your needs.
Condition is key, but even if you need to replace tires/tubes/brakes, you should be able to be up and running for $150 to $250 at the most.