A basic single speed could be built for under $800. $309 for the bike, $300 for the tank, and $125 for the engine.
I spent a lot more and did a jack drive to a freewheeling tandem crank then finished everything of with a continuously variable planetary rear hub. To shift i just let off the gas so the freewheel engauges, then I can shift. I only need to touch the clutch to start and stop.