This is a 3D printable version of my sumobot design at http://sumobotkit.com. It was designed for Nodebots Day in Las Vegas, NV. We were looking for a low cost alternative to expensive robot kits. With a couple of servos and a microcontroller, this is easily under $50. If you need a version that can be easily made in bulk, try the laser cut version in the SVG file!
This comes in 7 basic pieces that snap together. There are a couple of options for the top to mount a pinoccio or arduino compatible board and a curved or flat shovel. Otherwise, print two sides and two wheels and one of everything else!
You'll need a 1/2" ball bearing to pop into the hole for the ball caster on the bottom of the front. You can stretch some o-rings over the wheels for additional traction. A 4xAA battery pack will fit snugly in between the tops of the servo motors and the top of the robot.
More information and some build videos over at http://sumobotkit.com