Decoder

I think you mean an 'encoder' -- and for 5 dollars, you'll probably need to build one. The easiest route would probably be to print your own encoder disk and use a Hamamatsu photodetector (around $2.00/unit). You can mount the disk on the inside of a wheel or on the motor armature shaft, if available.

There are other approaches available as well, including the use of Hall-effect detectors (around $1.00 or so each) and magnets. This approach works a bit better if the magnets are mounted on a disk attached to the motor armature (again, if your motors have one). You can mount magnets directly to the wheel, but you won't get great resolution.

I'm assuming you're using gearhead motors and only need speed (not speed and direction or absolute position).

Hope that helps -- m