I suppose for your idea, a private among friends event, a beetleweight might be an option.
Those cost roughly :
12 pounds or so for a pair of gearmotors.
18 pounds for a pair of 10a botbitz speedcontrollers.
10 pounds or less for a small batterypack.

And to keep it safe a 2.4ghz transmitter/receiver.
Costs about 40 pounds or less.

Check out some of the beetleweight buildthreads on here.

Biggest plus is that the parts are cheap and less fidly than servo's.