So, people can flame me, but I would say that getting one around $50 American, with a solid frame (no dents, no rust issues, a few acceptable scratches, with the striping and headbadge still present) would be an acceptable starting point. You'll probably pay around $100 American to get it up and running, assuming that it needs all the consumables. Labor, if not provided by yourself, would be more, maybe another $100. Desperate upgrades would probably be the front derailleur (often broken by merely looking at it), then non-steel wheels (the originals can sound like a small child scream-crying when the brakes are applied to them), then new tape (because that's cool) and your favorite saddle (because your butt). The cost of the upgrades are based on your willingness to pay for such things, vanity, and how much time you spend reading the propaganda of this forum.