Technically, any bike computer that generates a .fit file (I would assume) should be compatible with Strava.

The least expensive that I know is the Garmin Edge 200...which is $89.99. It basically records your ride with GPS, includes speed on the readout, has a lap feature, etc. Basically this would be probably 1 step up from just running the phone app to record your rides in the sense that it's a set and forget, doesn't rely on your phone battery, and has a display you mount on your stem that shows your speed, time elapsed, distance elapsed, etc.

I don't believe it supports heart rate or cadence...but if your goal is tracking, it works with strava, and once there the file includes things like speed, average speed, average power, power during certain portions, etc...