I have a Polar F4 Heart Rate Monitor and I love it. I got it as a Christmas gift from my boyfriend (as per my request--what a good boy
), but I know they run for about $90. It is--by far--the best little gadget I've ever had for weight loss and motivation at the gym.
It tells you your heart rate and calories burned (based on your HR, age, height, weight and gender)--and it's SO useful! In the grand scheme of things, spending $90 on a quality piece of fitness equipment that keeps you motivated and lasts for years is definitely worth it. I've spent more money on videos, books and magazines over the years that did me little good, than a HRM costs.
You should get one--I promise it'll be your new favorite toy (I've had mine for 1.5 years and it's MY favorite toy still for fitness!).