I'm building a remote-controlled robosoccer set just for fun. The plan is to have 4 robots controlled by 4 human players. Currently I'm not in the position to fabricate my own robot bases, so I'm looking to buy them premade. I want them to be robust to withstand rough plays, fast and cost <$100 each. And not too big either. 15x15 cm is probably the maximum size. Note that I'm not interested in any electronics.
I've been looking through all the major robot supplier website, but none seem to be perfect.