I don't know what the craigslist culture in your area is like, but original rigid mountain bikes from the 80s and 90s abound for just over $100. The selection of quality and size is pretty extensive. My current commuter is a 1987 Mongoose Alta that I have been able to use a great deal of original parts from (canti brakes, bottom bracket, rims, spokes, freewheel, headset, chain) everything else I changed because I had nicer things lying around, not because the original parts were beat up or ruined. Definitely have a look around the used market in your area, the options may surprise you! Good luck!

Cheers
lverhagen