Another option is the Arduino, which usually retails for around $50 online. You could purchase one of these boards and then interface it with the robot open. This should allow you to control a driving chassis with a relatively low cost control system. One thing to consider is that to program the arduino, you will probably have to generate new code as the libraries are different. Also, there is a limit of 8 digital input/outputs. But for a driving base, I think this would be fine.
Here's the link for the arduino:
http://www.arduino.cc/
And for the robot open module:
http://www.team221.com/robotopen/
Hope this helps!