Get a Cateye Astrale 8 computer. Performance & Nashbar usually have them for less than $30 bucks or so.
It's wired, which is derided by the bleeding-edge techies and those who care more about how their bike looks than how it functions. Frankly - wired means no interference from other wireless devices, only one battery instead of 2 or 3, and a cost that's less than half for the same features.
The Astrale 8 has cadence, speed, odometer, time (since last reset), distance (since last reset), avg speed (since last reset), max speed (since last reset), a digital clock, and an auto-start feature. It doesn't have: backlight, wireless, two-tire settings (to move from bike to bike), or other wiz-bang things that have been put on computers recently.
But for cadence work? It's great. Set the big display onto cadence (RPM), and the small one on clock (handy that it's not a performance measurement to distract you). Glance down every once in a while. If it's low, and you can breathe, shift to an easier gear.