This guy built his own Segway for what he says was about half the cost. My favorite line from his description:

There is no redundancy or backup system. It is not even robustly made. Loose wires literally dangle out the bottom. In the fairly likely event of the software crashing, a wire coming loose, a component failing, or the batteries running low, the wheels will stop and the entire kinetic energy of the system will be used to accelerate my head toward the ground.