Buy used.
I found a nearly new Moon Dog for $30.00. I don't like the green color. It had broken pedals, and a bracket in the seat is broken. But I really like it. It will be a good summer build.
Get several used ones cheap, and make one good one or more. Around here, one can buy fairly good cheap mountain bikes for about $10.00, if you can see through dirt, some rust, and scratched paint. I bought 2, at different times and places, Magna Oasis girls bikes. They were not used much, and the original tires looked like new. The chains were rusty, and the cables were messed up.
I used parts off one Oasis to fix a really nice "Silver Canyon" Magna that had been abused and neglected. The Silver Canyon is still a current bike, and costs about 140 dollars new in Fred Myers, a department store around here. It has a 3 piece crank, and a very stong frame with suspension forks in the front. I got the Silver Canyon for free. The rear wheel had bad sprocket cluster bearings and other things wrong, there were spokes missing from both wheels, and the cables and gear change mechanism, derailure, were toast.
Be careful, because many mountain bikes haven't enough space for an engine. My friend moved the frame tube up and converted one of those Oasis girls bikes to a man's bike. The engine will fit just fine on that one. He had a similar one that he did.
Around here, one can buy a complete cable kit to replace bad ones. Walmart sells them for about 5 dollars for the whole set. Walmart sells chain breakers for about the same price.
