A lot of my Puegeots, etc, I picked up for $100 or so, and most were basket cases, but I tend to have another $300-400 into then by the time I replace the bars (usually Nitto), levers, pedals, saddle (always Brooks), wheels, tires (usually Schwalbe), cables, pads... it adds up.

Otoh, it's been a great hobby and I've gotten tremendous enjoyment out of every aspect: planning the projects, buying or scavenging parts, stripping the frame, cleaning parts, building them up, making something really special (to me), then there's the riding. I know I couldn't recover my money out of a single vintage bike I've restored. But overall, it's worth it and far cheaper than a guy into cars.