You can find old violins at antique stores alllll the time, and its often hard to judge whether one is worth fixing up or not. Here's one I was tempted to pick up for $40, but I decided it wasn't worth it--I currently want to focus my energy on making an instruments rather than resurrecting one that's already been made--and not particularly well, at that.
There are though, those ones that simply need basic setup: fresh strings and a new bridge, essentially--maybe some update fittings (tailpiece, chinrest, etc.) and a good polish. And with a few hours of attention, who knows, that $40 violin may resell for $300-$2,000 depending on its origins, sound, and quality. You may never know until you put it back together.