This is my first robot. Arduino Uno based, although I will end up trying out a TI Launchpad brain as well.
Total - $38 - not including Uno and proto-shield.
Tamiya twin-motor gearbox, truck wheels/tires,round robot platform, ball caster - $25
Pololu TB6612 FNG motor driver and carrier - $9
Battery holders - $4
Cheap Pololu round robot chassis, Tamiya twin-motor gearboxes, Tamiya truck tires. 9V battery pack for the Arduino. 2 AA battery pack for the motors, although I need a little more juice. I may add another 2 AA battery pack and supply each motor separately, or use a 3 battery pack and see how the motors like 4.5V.
Ongoing project. Just about to shoot my first video of the robot in basic maneuvering. No sensors installed at this time.