I paid $40 for a Bridgestone mountain bike. I spent an ungodly amount of time literally dissolving the seat post in lye ($16.99 for the bottle of lye), and another big chunk of hours taking it apart and back together for a full servicing. The bottom bracket threads where shot so I spend $20-something on a thread less bottom bracket. And then I found another bike I liked more (Mt. Fuji) so I'm going to try and sell the Bridgestone for $50. It was worth it because it was fun.

Hobbies cost money. Any attempt to calculate a return on investment is just crass rationalization on our part. Even if you get lucky and snatch a rare bike for cheap it will never make up for all the nickel and diming all your other bike endeavors sucked up.