I think the only way to get a good estimation is to use a heart rate monitor. Figuring out calories burned is a tricky thing and depends on MANY different factors including everything that Kathryn stated as well as how hard you are working(RPE),heart rate, time of day, how long ago you ate something, what kind of cardiovascular shape you are in, how much fat mass and lean mass you have on your body, etc. Also, calories burned may vary workout to workout. For example, last week I did the Gauntlet and the heart rate monitor said I burned 667 caloried (and this is ALWAYS an estimation), but the last time I did it I burned 530 calories. Like everything with exercise, it is an individual thing and the numbers will vary depending on all of these factors.
Carolyn