I was at a CVS drugstore yesterday and I noticed a radio controlled car for only $10. I'm currently waiting for delivery of a battery holder, two motors, two wheels, a caster, and some motor mounts from sparkfun, and needless to say, they cost more than $10 combined. So I figured I'd give it a shot and buy one. The car is a Thunder Tumbler and it has terrible reviews on Amazon which is unsurprising. As an RC car it is pretty awful. The joysticks aren't analog so the motors are on full speed or off. It's nearly impossible to steer and it pretty much always does wheelies and spins out. I suppose that is the "tumbler" part of the name but the cheap plastic parts don't hold up well to the abuse.
None of that really matters to me though because I plan to strip it down and just use the motors and base at much slower speeds. It even has a nice little removable battery tray. I don't have any plans for the remote control or the existing motor control circuit but I will keep them around, they seem to be built with an RX-2B/TX-2B pair of chips and it could be fun/useful for some future project needing a simple wireless controller. I'm just going to remove the existing electronics and control the motors with an adafruit motor shield.
After a little googling this morning, I find that I'm not the first person to have this idea. You can see working examples here and here although I think I will try to use mine vertically, standing up on the wheelie wheel more like R2-D2.