I just want to clarify Net Sales so it's clear as to why it's different than Gross Sales.
You've probably seen commercials where at the end of it they say that if you can't afford the cost... get in touch with them, or perhaps when the doctor gives you a prescription they also give you a coupon or voucher that allows you a certain amount off the prescription price.
Net Sales = the Selling Price Minus the Coupon Value, so a prescription cost of $100.00 that has a voucher for $30.00 has a Net Sales of $70.00, which is what royalties are paid on.