Some time ago, actually about a year and a half, before I got back to Romania from US, I have built a sample robot, something similar to the $50 robot. Unfortunately it was written in Romanian so I would have to translate it to English. There are some pictures with building stages and schematic in Eagle, code in Basic, a sample video. I think it turned out OK, but if I would have to do it again, I would use a different (cheaper) microcontroller and get the whole project to about 10 bucks. Well, there is some dumpster diving necessary... to find a couple of VCRs to cannibalise for parts.
For the people interested to see the pictures and the video, check out this
link. Please scroll down about half page to see the attachments...
If needed, I will translate the tutorial in English for you (need space to put it up).