Let's Make Robots!
tc  

The goal here was to create a as functional a Propeller robot as possible for less than $50.  The Bluetooth capability pushes it over the $50 mark, but I thought it was a feature that was worthwhile for someone beginning in robotics.  I was able to adapt a mini-BASIC to run over the Bluetooth connection which accepts BASIC programs like this one.

10 FORWARD 150
20 PRINT HCSR04
30 IF HCSR04 < 150 THEN BACKWARD 200
30 IF HCSR04 < 150 THEN RIGHT 250
40 GOTO 10