Well the first possibility if you want to build it yourself is the $50 robot micro controller. it is really cheap to build and a programmer is cheap if you have a serial port, but they are quite old and there are not many left especially on laptops. therefore you may need to invest in a programmer but the way to look at it is it is a one time buy and you can get one that will last a long time for $20. of course there are more expensive ones out there that can do more and are better quality but you can get some nice ones to start with that are cheap.
the $50 robot is a fantastic starting point for robotics, how many people on this forum started with hibby robotics.
Do you want to build a microcontroller from scratch or want a ready made microcontroller?