My best was a Motobecane Super Mirage frame for $60 that I bought as a complete bike for $25 and used the wheels on it for my fixed gear schwinn. So, not a bunch of money made but I think it's the most I've gotten out of a bike for the least put in. I also sold a $27 3 speed for $130 but I put many, many hours into it, full oxalic acid bath, full rebuild, the works. I recently sold a $35 centurian for $160, but I had to replace the fork for $50 and bought new tires for $10.