The board for the $50 robot is fairly inexpensive, you can swap out the chip easily if you fry it (especially if you use a chip carrier), you learn a lot by making the board, and its a decent basic board. The next step up from there, or first step if you would prefer an already assembled board, would be something like the Arduino. Personally, I use the Axon II and am very happy with it. It is more expensive, but its a board that you can use later on for more advanced projects. The nice thing about these options, is that any code you build for one can be very easily adapted to another since they are all in the same chip family.