Banebots just came out with some new wheels, apparently.
http://banebots.com/c/WHB
$14 will get you a 4" wheel with whatever durometer you want (30, 40, 50) and hub to mount it on a typical keyed shaft. They also have a wide hub so you can double up two of the wheels. These look really neat, has anyone played with them yet?
EDIT: you can also just get a keyed wheel for $5! I really want to try them out.