At face value your idea sounds like an interesting way to fill your class with participants. As many of the other responses state, however, I feel as if there’s a bit too much room for controversy later on down the line.
I’m interested in how you might advertise any type of class like this correctly. Thoughts?
Instead of the variable being “pounds lost,” have you considered basing your pricing structure on how much a participant’s body fat percentage decreases?
Assuming your measuring device is accurate, there would probably be less of a chance of unhealthy actions taking place. This would also go hand-in-hand with you properly educating your exercisers on the physiological differences between “weight loss” and “decreasing body fat percent.”
Another idea would be to market your group fitness classes as unique 10, 21, or 30 day challenges. Here’s an awesome link for more insights on how you could do this: http://www.ideafit.com/fitness-library/the-10-day-program
Finally, here’s a link to a previous question whose responses might be of interest to you: http://www.ideafit.com/answers/anyone-have-ideas-for-client-incentive-id…
Best of luck and I hope at least some of this is useful to you!