How do I make/get a odometer for a lunch box sized robot?
I had the idea to use a computer mouse (optical) to do this - cheap, easily available. You would need two, if you want to be able to measure turning rate, one on the left and one on the right.

You don't even need the whole mouse - if you pull it apart you can ditch the case, the buttons, and about half the circuitry. You will need to do some hacking to figure out how it works, but I doubt you'll find a cheaper odometer that is anywhere near as accurate.

Note: I pulled one apart, then got distracted on another project, but I can't think of a reason why it wouldn't work.