If you can't afford a wireless computer - then you need one that has a wire long enough to have the sensor mounted on the chainstay (the horizontal tube the goes from your pedals to the rear wheel). Then it depends on where you want the computer itself.

For example, on my wife's mountain bike, the computer had to be mounted on the top tube (the tube that goes from your handlebars to your seat) in order for the sensor to reach the rear wheel.