In most markets, you can find excellent mountain bikes with low miles for $75-150. Garage sales are often less expensive than craigslist finds. Most used bikes need new cables and cable housings as a minimum, due to stretching and lack of lubrication. Minimal cost but well worth it. If you have an REI in your vicinity, there shop rates for a tune and cable swap are very reasonable. Also, many here are fastidious about cleaning and regreasing the wheel bearings, and on older bikes without cartridge crank/bottom bracket spindles.

If you have time and minimal mechanical aptitude, you can read up here and do the work yourself.

Enjoy!