Most microcontrollers are under $10...
The ATmega 8, which was used for the $50 robot, is available from many suppliers(Google ATmega8)
Have you got the electrical components ready to solder?
Or do you want to buy a pre-made microcontroller?
If you want a pre-made microcontroller most SoR members suggest the AXON or Arduino.
If you want to make your own microcontroller just follow the $50 robot tutorial, it's pretty straight forward.
Use the ATmega 8 for the $50 robot, upgrade to 168 or 328 later!