lidl have inexpensive heart rate monitors in this week, just under £20 I think. OH bought one on hols from decathlon, they have a few branches in the uk, or online. His was 19 euros, again just under £20.
Think that is about the cheapest you will pay for a decent heart rate monitor. You need one with a chest strap I think, without it your heart rate won't be constantly monitored and so you won't get accurate cals burned data.