I think it really depends. How much is it, and what do you get for it?
For example, if the fee gets you a special demo team uniform, lessons from a professional gymnastics instructor on how to do fancy stunts, transport to demo locations in a school van, boards and tiles to break, special equipment, water, food, etc - I think it's reasonable for them to ask for something. ($30/month seems like a bit much to me, though.)
Demos also usually cost the school money - one local festival I was looking at wanted us to pay $300 to come entertain their audience. That's higher than normal, but $50-100 is typical in my experience.
Now, we don't charge for our team, other than for the team uniform, but we do expect students to transport themselves, to help setup and breakdown, to bring water and snacks, etc. And it does mean we're giving 90 minutes of extra instruction every week in exchange for a small amount of publicity a few times a year.