I bought a $40 heart rate monitor a few weeks ago, and I will say that it's a GREAT investment. When I got my fitness assessment, I was given a heart rate range that I should maintain while exercising. It's very revealing to see where that rate is when hooked up to this device. I reach my max a lot faster than I thought.
Here's another interesting fact: My heart rate gets to the same level with a brisk walk as it does on the ellipcial trainer - showing that yes, a brisk walk is just as good for you from a cardio-vascular standpoint as time spent on a machine.
I will continue to visit the gym (and the elliptical) (and the weights) a few times per week, but it's good to know that a walk in-between is just fine too.
I was also told that a $40 monitor is pretty much just as good as a $100 one. I bought the Polar one with the EKG quality monitoring. All you really get for more money is more features. I'm barely using the features on the one I have, so I'm happy with it.