yeah, overthinking it just makes it less fun. why not: old clunker (we all know what they are without getting too specific) that's total "build" cost is not more than $50 including initial purchase.
Build cost does NOT include the time put in by participant, but does include equivalent value of parts, supplies, and time and access to specialized equipment.
that way if you are a parts hoarder - even if you got the parts for free- you would have to include fair market value (or a bit below for quick sale!) for the part you used.