christoph, good to know about these low cost alternatives. The original reason I bought a Garmin 500 a few years ago was that it was the cheapest option I could think of which will allow me to capture telemetry data of my rides not just for training reasons but at the time I wanted to superimpose telemetry data via a home brewed app I was programming, on cycling videos I was capturing with my GoPro:
http://youtu.be/Aw_I_c3iYfc
The 500 was one of the cheaper options that allowed capturing of GPS along with heart rate and cadence information. As mechbgon mentioned it is relatively easy to get additional attachment heads for the Garmin 200/500 devices and attach them to the handlebar or stem of your bikes. The GPS unit can easily be removed from the bike by a quarter turn.