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).