Large metal-geared DC motor
340RPM at 12V. D-shaped shaft, suits our yellow 65mm wheels (but requires some force to mount).
The speed of brushed DC motors is approximately proportional to the voltage you put on them. Double the voltage can give you double the speed, but beware of issues such as torque limitations, overheating and faster wear.
Price: $14.00 (for each Large metal-geared DC motor)