I have a SunDing Bluetooth speed and cadence bought from Amazon for $12. Even though it's cheap, it works great with Runtastic Road Bike Pro app on the iPhone ($4). I also have Polar HR paired with the same app.
$12 SunDing (Amazon)
$10 generic iPhone holder (Amazon)
$40 Polar HR (eBay)
$4 Runtastic Road Bike Pro IOS app