The amount of calories burned in class depends on a few factors, including: weight, size, sex, fitness, amount of muscle mass, pregnancy and effort/exertion.  Most sources agree that Bikram is the most rigorous...

...form out there and give a general range of 600-800 calories per class.  Iyogalife offers a comparison of caloric breakdowns for various popular yoga styles and lists Bikram at 636 calories per hour of practice for a 140 pound woman.  That's 954 calories per class!