Robot Positioning Systems?

CyberLeader asks: "As a hobby I've been developing a small robot to mow the lawn (giving me more precious hours to read Slashdot, of course). The intended brain of the 'bot is a small Linux-based on-board PC. To be able to operate the 'bot as desired, the PC needs to be able to accurately identify the robot's location to within about 10 centimeters, but I can't find any decent system to do this that falls within my cost constraints of a few hundred U.S. dollars. The position doesn't have to be global; it can be relative to radio beacons or similar devices placed around the yard. Any suggestions?" Ah, the mischeif I could have caused if I had had one of these as a teenager...