How to Find a Sale Price. Retail stores offer discounts on items as a percentage off the original price. The discount means the store is offering "x" amount of money off the original price of the item. There are several ways to calculate the sale price of a product. In a mathematical equation, one way to calculate a sale price is original price - discount = sale price. Finding the sale price of an item isn't difficult and knowing how to find a sale price of an item can save you money.