I think the Rexs Challenge rules would provide a good starting platform for a lower power/cost/destruction featherweight category- although a few mods to the rules may be beneficial.

Something like:

DRIVE MOTORS - must be ex-car windscreen wiper motors. You can modify the motors however you want. Include drills (possibly DeWalts, Im not sure on that matter) and gold motors as forms of drive - since watching two slow WW bots can become a bit boring.
PNEUMATICS - for flip-ups, self-righting mechanisms and other weapons are encouraged. All systems must use a maximum of 50psi air pressure. Increase the pressure limit to 150 p.s.i to allow for proper flips rather than the tip-over style flips that are usually the result of 50 p.s.i flippers.
Possibly include spinners but either limit the revs or the motor choice.
Then include the power light, removeable link rules etc.

That may provide a base for working on but thats only my opinion.

Feel free to deliberate

Regards,
Lenny