There are so many variables to consider.
1. How far do you have to travel?
2. What are the expenses related to workshop?
3. What do group fitness instructors earn hourly in your area and what do presenters charge for workshops in your area.
4. The fact that you have been singled out to presents is indicative that have value to them.
5. Is it you who is filling the room with 30-50 persons? Are they paying specifically to do your class?
It is difficult to say. I hope this has given you some direction though?