If youre strapped for cash, and still want to build a robot, then startermotors arent realy the way to go.
For the same amount of money you can get some wipermotors. With 4 or 6 you can get a heavyweight moving, giving more controle and upgrade possibilities in the future.It wont be fast, but you can use smaller cheaper batteries and due the low speed On/off relay control can be used.
And speedcontroling startermotors was a disaster, as my first robot used a startermotor, so I speak from experience.It isnt cheaper.