You are way out of the "simple computer" category when it is based on GPS readings instead of using a wheel magnet. The description given by mdarnton is for the Sunding wired computer which is available for under $10 from numerous sellers. I've given away several of these as gifts. They are accurate as long as you correctly enter the wheel circumference and will go for at least a year without changing batteries even with daily riding. It would be a minor miracle to find anything that cheap that does not use a magnetic sensor.
Here's the red dot mentioned above.
