There are a lot of things it depends on, how fit are you? How much do you weight, what was your heart rate the whole time, how much do you work out on average? Have you hit an exercise overload, where your body actually slows down what it burns? These are just some of the few things you need to know. Heart rate monitors are great to tell you how long your heart as stay in one zone or another, but basically you got to watch the monitor more then the video as the moment your heart rate drops, you burn less calories, and if it goes up you start burning more. And if it goes up for 5 minutes then goes down then back up, you got to factor that in as well, because when you do that, you don't burn a steady amount, and all this is just for a very rough estimate, as you also got to take into consideration the exertion of the muscles, and how much you move them because moving them also burns calories, not just making your heart pump faster. So you got a lot of adding to do. Basically this is what I tell most of my clients, if work out for an hour and your trying to lose weight, consider you burned 100 - 150 calories no matter the workout. If you are trying to gain weight, consider you burned 200 - 250 calories.
For the dieter, if they burned a little bit more, it's okay as long as they don't go trying to chop a huge portion of calories that they need out of their diet, if you are exercising and dieting you should never deduct more then 200 calories from what you need to maintain your current body weight.
Now if your trying to gain weight and the workout didn't burn that much, you'll gain a little bit quicker but not enough to really notice, as it takes a lot of calories to make one pound of fat. You still will still have to eat right and more calories to gain weight.
I hope that helps.
Kit