49 cc Hua Sheng / Grubee mountain bike - $500 - Seattle

See "Helga" in the picture gallery under (owner Vishnu Tensleep). The engine / gearbox kit costs $390 plus shipping and tax on bicycle-engines.com. I've mounted this kit on a Trek mountain bike to which I added a shock fork, real motorcycle throttle, fenders, etc. The engine has about 400 miles on it, with the oil changed twice so far. Basically, the price doesn't quite cover the parts let alone my labor. This bike is operable as-is, nothing needed to get you on the road. I was using up until this week when I bought a Honda Metro scooter. You could go cheaper and get a 2-cycle, but if you want a 4-stroke this is a good price.