My latest hobby has been finding and re-building old bicycles. My rule for myself is I can only spend up to $30.00 for the bicycle I will be working on.
I try to keep total costs under $100 after parts and such.
My most recent two were both on the side of the road for free or headed to the landfill. They were rusted and needed to be completely stripped down. I added new rubber and plastic parts. Everything else was sanded and painted.
I also removed the front derailleur from both bikes and made crank-gear a single gear, so each bike is now 6 speed.