I need some advice on a costing structure for a new after work small group fitness class for a company. I don't know how many

I don't know how many people will participating yet, nor do I know many other details. This is all new and the HR person asked me to put a proposal together. The company would be paying for it, not the individuals, as far as I know. I need advice on how much to charge without low-balling and undervaluing my time and services, but not too high, where it is cost prohibitive and will price myself out of the deal. I was not sure if I should just charge a flat fee, to make it easier and for their budgeting purposes, no matter how many people participated, or if I should have a minimum amount, just for showing up and then have a tiered cost structure based on a range of # of people. Here is one of my potential scenarios - 0-5 people - $ 100 min. to show up and anything over 6 people, an extra $ 10 per person. Another scenario was 0-5 people - $ 100 minimum, and then 6-10 people an extra $ 10 per person, and 11-15 or 20 people an extra $ 5 per person.

I don't know if you think the minimum is too high, but I need to consider my time to bring in my equipment and set up and break down. Also I will probably need to purchase more equip (bands, balls, cones, ladder, etc.) , because I don't have that much right now. My time to plan the workouts, drive time, etc.)

Any advice or help would be much appreciated. Thanks!