I use the "stationary bike" options and try to find the ones that most closely reflect the calorie burn for the program I am on. The intervals program on the ellipticals at my gym (at level 10 of 25) has a calorie burn of ~730 calories per hour so about 12 calories per minute. That is about the same as the "stationary bike: vigorous effort" option.
I guess if you know the wattage of the excercise and your weight you could work out the calorie burn... I might look that up...