We recognise and appreciate different situations. You may have dancers in a single routine and others in multiple ones. We use the number of dancers as a formula, but you are welcome to develop a policy for how to allocate the fees.
One option is to break dancers into three groups. For this example, let's say we have ten dancers so that the total will be $150+tax. Let's use groups of 1 to 5 routines, 6 to 10, and 11 plus. Dancers in the bottom group pay $10+tax, middle group pay the $15+tax. By just adding up the total for the first two you can subtract it from to total. The difference, divided by the number of dancers in the 11+ groups is what they pay, which in this example would be $17+tax.