Best fitness tracker compatible with WP ~50$?

As usual, after the holidays, it's time to get back into shape. But I would also like to exercise optimally, and am looking for a fitness tracker. In essence, I actually only need something to help me with the running. So basically heart rate monitor.

What are my options if I want it to be supported with an app on WP?

Sports tracker, adidas, something else?

I have found a few, but they seem to be quite pricey, and I do not see it so necessary as to pay more than 50$ for it.