I am trying to figure out a list of formulas and have it all figures except;

I need to be able to add and calculate units by thier commission.
Say if someone sells 10 units they have $50 commision on each

And if someone sells 11-15 units they have $50 commision on the first 10
units and $75 commision on units 11-15

And if someone sells 16-20 units they have $50 commision on the first 10
units and $75 commision on units 11-15, and $100 commission on unit 16-20

And so on, I need to figure what the commission is on each unit and add them
together. An example is 10 units is $500, 15 units is $875, 20 units is $1375.

Any help is greatly appreciated.