If you use the wheels then use the price paid for the wheels. If you use the tires without the wheels (but you purchased the wheels and wouldn't have received free stuff otherwise) I would either - fair market value for used tires (what, $5 each?) or - as a portion of the sum paid for the wheels.

The idea isn't to nitpick as to how to extract the most value based on the rules...it is more...what is the best or worst $100 bike I could build if I didn't have 10000 bikes in the backyard and a Chernobyl of parts in the basement that cost me nothing at this point.