I assume that the classes are offered free to members of your club because else you would know that number. I also assume that you track the number of participants per class.
Maybe the best would be a member survey where member can indicate the importance they attach to each of the services you offer. I would ‘sell’ the survey as a way to serve the members better. A question could be to ask what services your members are actually using. Another could be to sort your services in order of importance (I would allow equal ranking). Yet another question could be whether there is anything the members are not using at all.
While none of that will leave you with an actual number it will give you an idea of the value your members attach to your classes.
In order to motivate people to actually take the survey, I would offer a drawing among the participants; eg. three months free membership. A water bottle will not do it.