Posted: 04/07/2013 at 16:47
If you want the cheapest solution, then you want to get a HR Belt that will work with your iPhone. This means it will need to be bluetooth, and Polar make the H7 bluetooth belt for about £50.
A dedicated GPS watch with HR belt is a popular option, such as the Garmin Forerunner range. You should be able to get a forerunner 110 with HR belt for £115 from amazon (maybe cheaper else where). The HR belt with the Garmin uses the ANT+ technology to connect to the garmin watch, so this belt won't connect to your iPhone.
With the garmin, you can download the data from the watch onto your computer, and then synch this with web-based training programs.