Digikey does sell some
Rotary Optical Encoders. These are single-piece enocders for motor-monitoring applications. Up to 3,000 RPM. You won't be able to use them on the robot controller though, as it is far too slow. You'll need to feed the data to a separate microprocessor and use that to count shaft rotations, then feed the data to the robot controller.
Be careful when devising a way to mount them. This
Bournes Data Sheet says it doesn't tolerate much axial stress. You might snap it off.
You could contact
Bournes and see if they'll send you a sample. You'll still have to include the $50 price tage in your $200 total (that eats up $100 there -- one for each side), but it'll possibly save you some money.
Basically, what I'm trying to say is... this is an option, but it's not for the inexperienced.