Its a tricky one

I was thinking on limits by the types of materials e.g. I could get a chunk of titanium free (I wish) then spend a few hundred pounds on machining

Valuing anything is subjective, especially robots with a small community

Limits around motor power output , battery capacity , materials (e.g. only Al, HDPE , mild steel - nothing beyond a jigsaw and B&D workmate) would probably be needed. The downside this would exclude some dumpster finds but it has to be a simple set of regulations for people to understand and build to