It does sound like a good idea at first.
It might have merit, but usually things like this can go sour. Companies have had programs like this in place- incentives for weight loss. The problem is that sometimes people can take it too far. For example- some people may try unhealthy routes to keep coming to your classes by starving themselves.
The other problem is how often will you weigh people?
It’ll be time consuming if you do it every time, and people’s weight fluctuates by several lbs every few days.
If you do it once less often will it be enough to be worth it to the members?
If you do it will you have to bring in more people to reach your desired bottom line?