You should be able to get a used laptop (for free) with a serial port on it.
The new Jags (are they still $80 with an FRC discount?) will accept a serial input, and have inputs for pots and encoders built in to them.
For something smaller, check out ROBOT or SERVO for servo controller boards that provide similar function, but for multiple servo motors.
All the laptop needs to do is send serial signals to the Jags (or servo board) saying "go to this position" or "rotate at this speed".
You might want to check out
http://www.robotsee.com/ for some interesting options... like using an old Android phone as your controller, or tying in a webcam or joystick to that old donated laptop.
For the ultimate in "cheap" though, you can run two 1 amp brushed motors off an L293D H-bridge chip (a buck or two, it's "big sibling" 2A version the 298 is just a bit more) and control it all with an Arduino or PIC.
Jason