The fee is so very locale specific that it’s difficult for someone not in your area to give you an adequate answer. In addition to basing your pricing on where you’re located, other factors such as your experience level could effect the amount charged. Lastly, I’m not sure how others price for group exercise, but I start with the amount that I need to make for the class, then calculate the minimum number of participants for the class (which gives me a basic idea of how much per person), and then designate a maximum number of participants. With that method you COULD price classes something like this: 3-6 participants $X per person; 7-10 participants $Y per person, and so on (sort of a sliding scale).
I hope that this helps.
The answer to your question depends a lot on a couple of factors: they type of group class that’s involved, and the degree to which the instructor is an ‘active participant.’ For example, I train teams and small groups of athletes. Depending on the actual session involved (e.g. is it a weight training session, or something like a plyometric or speed training session) my active involvement varies. If I were to try to actively participate in several plyometric training sessions in a week, I might find myself injured, whereas if I’m merely supervising and/or demonstrating a weight training session, I may be able to do several of those in a week.
So, take a look at what type of group sessions you’re talking about and see where that answer leads you.
Good luck, and stay healthy!