The CatEye Velo 2 is less than $20, the only function it is missing is max speed (but I've gotten over it), and it has only one button. Plus, you can get an adapter that lets you mount it on the stem. It does make a slight tick sound every time the magnet passes the sensor, but I don't hear that when riding.

I saw something last week about some "Italian sports car bike mirror" or something like that, it was very inconspicuous, inserted in the handlebar end, and SEEMED like it would work pretty well. It was about $20 after shipping.