Not sure how many this will appeal to, I'm still yet to decide on it myself,
but seems good if used with Amazon credits.
Wahoo Blue SC Speed and Cadence Sensor for iPhone, Android and Bike Computers
rack and capture real-time speed, cadence and distance on your iPhone, Android, and bike computer.
Connects wirelessly via Bluetooth 4.0 and ANT+ to your iPhone, Android and/or bike computer.
Works with Zwift and TrainerRoad! Capture cadence on your favorite indoor cycling platform.
Also works with the free Wahoo Fitness app, Strava, RideWithGPS, MapMyRide, Cyclemeter, and more!
Lightweight and easy to install, comes with both rubber band and zip tie mounts. Universal fit - works with most bikes and frame sizes.
cheapest it been on Camel
http://camelcamelcamel.com/Wahoo-Cadence-Sensor-Android-Comp…
Or $12 delivered for a clone on eBay. Works fine with a Wahoo computer, IOS, or Android, using BT-LE (no ANT+)
e.g. http://www.ebay.com.au/itm/162070592341