I'm 30/f, 5'7", 128
I'm really not all that bright. I've been using a Polar FT7 since September and have been able to figure everything out without trouble. I give it high grades.
I have a polar F6, and like it. In it's simplest mode, you just enter weight, height, and age. It was less than $100 (on sale).
HRM's use your weight, height, and age to estimate the various technical parameters that are actually used to calculate your caloric burns. If you happen to know any of those other parameters (rest heart rate, max heart rate, VO2max etc.) you can enter them manually, and I like that. You don't have to know them all. You can enter just the ones you know, and it estimates the other ones.
The fitness zones work in a similar way. There are pretty good default zones that are generated from your basic data. Or, you can go in and manually set the zones to whatever ranges that you like.
The HRM system is the combination of a watch and a strap around your chest. The strap just transmits the heart beats to the watch, and doesn't do any calculations or have any memory of its own. The straps also transmit to many cardio exercise machines, and your heart rate is automatically displayed on those machines, even when you aren't wearing your watch.
The cost differences are mostly in the watch, and how many functions it has, and what it does with the signals from the strap. Mine shows the time when I'm not using it in heart rate mode. It shows elapsed time, like a stopwatch, during my workouts. It continuously displays my heart rate, and calories accrued, while I am working out. It keeps track of my average heart rate, maximum heart rate that I hit during the workout, and the time in one large "exercise zone" that I set ahead of time, and the time in each of three different zones of intensity.
What mine doesn't have is the ability to download the data to a computer. That is very useful for certain fitness training aspects, but not needed at all otherwise. Also, there are some other type of heart rate monitors that, from a purely calorie counting standpoint, are supposed to be better because they also work at light exercise levels, and for anaerobic activities. But, they aren't compatible with the exercise machines, and are much more expensive. They haven't been out long enough to be well proven either.