Fireflies in a Jar
This isn’t a robot but it was still simple and fun to build. I sent it in to Instructables and ended up[ getting a most popular award for it. You can see the full Instructable with all you need to make one by following this link.
It took me about two hours and didn’t cost anything out of pocket because I had the parts to make it in my collection of crud. However, I assume that if one was to buy the parts needed for this project, the cost would be under $20. NE-2 neon lamps are cheap, often found on sale at 10 for $2. The most expensive components are likely to be the capacitors which could be as much as $1 each. The resistors maybe $2 for all 10. A power cord is often found for $1 and the pair of diodes fifty cents. Actually, you only need a single diode, I just added the second one for a clipping effect.
Check out the instructable and be sure to read the comments. There is information on relaxation oscillators and variations of the project contributed by others.