Josh, my favorite build so far is a HF on a Kmart $200 riverside
I made the mounts with a drill press, old kit parts and a former friend made the gear reduction. The pipe is electrical conduit and a plumber's rubber blow bag as a muffler.
The engine was $99, muffler $12. Mounts around $10. Kit parts I had on hand, figure 40 or 50 bucks.
so engine/drive train, around $170. Bike $200. Total, $370 or there abouts.
Seen folks build with dumpster dives for $0. Some uber cool builds for around $1,500