I have two indoor cycles (aka "spin bikes") in my home gym, each with a different wired computer to track cadence. One is running a Sigma 16.12 which was about $28 from Amazon's Warehouse Deals section and the other has a Cateye Strada Cadence that I picked up off Ebay (used) for about $12 plus another ~$20 for a new wiring harness and mount.
The Sigma has more functions and was slightly cheaper but the Strada is much easier to setup and use, and Cateye replacement parts are much easier to find. If I had to buy another right now, it would be another Strada Cadence.