what is a good robot kit to start out on?

im wondering if there are any build-your-own robot kits w/ instructions. something that is for beginners, but not something thats ridiculously easy. my budget would be $200~300, or maybe a little more if i need it.
it would be nice if i can learn the basic ideas of how robots work.

http://www.vexlabs.com/?gclid=CIOdj8fDwpECFQPplgod4yJxDA
http://www.electronickits.com/robot/robot.htm
http://www.hobbytron.com/RobotKits.html
http://www.sciencekits.com/robots1.html