\$275 from the MakerShed.
With the Arduino Robot, you can learn about electronics, mechanics, and software. It is a tiny computer on wheels. It comes with a number of of project examples you can easily replicate, and it is a powerful robotics platform that you can hack to perform all sorts of tasks. The robot comes with a large number of inputs; two potentiometers, five buttons, a digital compass, five floor sensors, and an SD card reader. It also has a speaker, two motors, and a color screen as outputs. You can control all these sensors and actuators through the Robot library. There are two different boards on the Robot: the Control Board (top) and the Motor Board (bottom). If you're just getting started with electronics and programming, you should work with the Control Board. As you become more experienced, you may want to tinker with the Motor Board.
More info at: Arduino Robot