There are some super cheap computers on sites such as suntekstore.com and dx.com. I've used them, and they work fine. One I had even had a temperature readout. Bike computers were invented in the late 70's or early 80's. The kind that use a magnet and sensor haven't needed to evolve, so how to make them is well known. The chips and displays must be mass produced. This is why they work so well and are so cheap. Really, any of them will do fine. I've gotten some for as little as $6.

Feel better!