http://182.160.128.150/Astro/Controller/
The main cost is the Arduino itself - $40 or so from Jaycar for an Australian designed clone. The Arduino is overkill, but its handy and flexible and perfect if you dont know what it is.
The driver IC's are $4 or so each. Everything else is relatively cheap bits of hardware - The panel-mount USB socket is probably the most expensive thing after the Arduino!
In the images shown, i have it set up with the connectors and IC's etc, to drive three motors, so to drive just the factory motor would have substantially less wires, etc needed. I have also eliminated the need for the power cable to the motor housing.
Adding a hand controller to this configuration would be trivial as well - essentially just the cost of the buttons of your choice and some wires, resistors and a case.