Tamiya is a Japanese company that makes many hobby-type products- model kits, "experiment" sets, RC toys, and others. Deeply buried in their product line is a set of generic motor/gearbox combinations.
These range from very simple to very complex; I have a couple of their high-power ones, and one of the 6-speed ones as well. They are well constructed, fairly tightly toleranced, and loaded with features. The have a twin-axis gearbox, a worm drive gearbox, and a planetary gearbox, in addition to more "standard" types. Typical cost is between $10 and $20, depending on the model.
One of the most enjoyable items in the hobbyist robotics arsenal is the RC servo. However, most servo motors tend to be either pricey or weak, and they tend to be non-reconfigurable, meaning that you can't readily trade off speed for torque or vice-versa.
To that end, I'd like to make a small PCB that would let me take an input from a shaftless potentiometer attached to the output shaft of a Tamiya gearbox and turn the gearbox into a servo. Target cost would be $10 or less in parts, because that puts a Tamiya gearbox + this circuit in a price range to be MUCH better placed than a comparable servo.