The thing to look for with wireless is some kind of encoded signal (typically referred to as "digital wireless"). Without that you end up getting interference from overhead electrical wires and such nonsense. I used to have a cheap wireless computer, and I found a spot where I could hit 40 mph while standing still.

I've got a few ANT+ computers. I like those because the sensors connect using a standard protocol, so you can mix brands between the sensor and the computer.

You can get a Giant NEOS ANT+ computer that has a backlight for $70, but that doesn't include the speed sensor. I was able to get a couple of Motorola speed/cadence sensors on eBay for $15 each a while ago, but they seem to be gone now. A few of the Asian sensors available are ambiguous as to whether they are ANT+ or Bluetooth. Maybe they do both.