You might want to look for a used steel rigid mountain bike from a name brand bike manufacturer, watch Craigslist. Lots of them were sold in the 90s and you may be able to buy one in decent shape for $100 or so. Get new tires and tubes, and even pay for a general service/inspection at a bike shop is maybe $150 more (or less if you can do the work yourself). You would then have a good quality bike for $250.