Dive into robotics

I have been taking a robotics class and have fallen in love with robotics. I am currently planning on teaching a robotic class for High School Students this summer. I will follow up with more information about what I am playing with for this class:

Explored:
Beetle Bot (6 direction with use of H bridges)
Hex Bug (2 direction)
RC car bot (6 direction but H bridges come for free!)

It turns out RC car bot is the cheapest at $5.77 from walmart who can beat that! I'm planning on using Atmel MicroP's in the class with the AVRUBD firmware. My goal is to build simple line following robots for <$15 a piece.