I bought an old Huffy w/3-speed hub and a complete, nonworking Bike Bug engine for $35. It's a low-priority project now.
But my best find has to be my 2000Trek bicycle for $200. The owner bought it new 10 years ago for $800. It had sat neglected these past three years.
Mounted on it is a Staton gearbox, a NuVinci hub and a Mitsubishi TLE43 engine (not broken in yet). I couldn't find a ride to the seller's house, so I told him I'd buy the bike (sight unseen) if he brought it to my house. The seller trucked it over to my place, after pumping up the tires and putting fresh gas. After paying the man, I fired it up, then rode to the adjacent park. That bike climbed up the steep grassy knoll effortlessly.
I WAS gonna sell it to another member. He hasn't called back, so it'll be my "back up" vehicle to commute to work.