Firstly the calorie count is not 100% but using heart rate its indicative - works better for aerobic type exercise, anything with weights wont work very well.
What exercise will you be doing or where will you be wearing it?
some need a chest strap and some just a wrist watch.
ASDA do a basic HRM for £19 - its just like a watch
Polar do a FT7 which records details and comes with chest strap, but has a zone reading to tell you if you are exercising and burning fat. £70 ish (I have the FT4 - does what I need)
If your wearing it all day and want something more watch like the Polar F6 is ok and for girlies only, so I wont be wearing one - £70
Oregan scientific PM25 is good at £39 has a 2yr warranty
This piece might be useful in taking your decision -
Heart Rate Monitors Review
Enjoy!