At the other end of the cost spectrum I've just ordered one of these
http://www.wahoofitness.com/Fisica/Wahoo-Fitness-Fisica-Fitness-Sensor-Case.asp
It's trick is to act as an ANT+ sensor, so that your phone can grab HR, speed, cadence and power from sensors.
I'll report back when it's delivered and I've tried it out...
OK its arrived, and its really rather good. Quite large, but fits on either bars or stem, and either portrait or landscape. Seems very robust and probably waterproof. They promise an add-on battery, but in the meantime you can charge via its mini-usb port.
ANT+ works very well, although ironically I haven't found a bike computer app that I like that supports it yet.