Cheap cycle computer with rear wheel sensor?

RichMTB Posts: 599
edited February 2010 in MTB buying advice
Hey Folks.

For Sport Relief I've been roped into a marathon turbo trainer session outside my office.

In order to track how "far" I've pedalled I thought a cycle computer with a rear wheel sensor would be a good idea but all the one I've seen with this feature tend to be on the expensive side.

Does anyone know of any cheap alternatives, for instance could I bodge a wired computer to run off the rear wheel instead of the front one?
Step in to my hut! - Stumpy Jumpy Pacey