If you want cheap wheels, go and buy some adult-size rollerblades. The wheels are strong and are designed to take the weight. With a rubbery, sticky grip and generally not far from 10cm in diameter, it's good deal, especially since you often get 4 wheels on each blade, making for 4 two wheel drive robots or 2 four wheel drive robots.

The only downside is that they are quite skinny, and very little rubber actually touches the ground (otherwise, whilst skating, there would be too much friction for it to be effective), meaning the grip wouldn't be incredible, but for a flipper bot or something, they'd work great

Wow, I wrote more than I thought

Ellis