"What makes the robot wicked fast is the fact that it's got four independent drive motors, each one of which has a power to weight ratio that's absolutely banan

"What makes the robot wicked fast is the fact that it's got four independent drive motors, each one of which has a power to weight ratio that's absolutely bananas. Only 6 millimeters in size each, the motors output 1.5 watts of power at 40,000 RPM, driving the individual whegs through 16:1 planetary gearheads. They're not cheap (hundreds of dollars each), but they make for one crazy little robot. And of course, independently driven whegs make the robot smaller, lighter, simpler to steer, and generally more efficient overall" What are my current alternatives if I want to build something similar and I'm only willing to spend say $50 per drive motor?