How the discount impact works
Formula. Extra sales needed = (profit per unit before ÷ profit per unit after) − 1.
Example. Price 10, cost 6, 20% off. Profit per unit falls from 4 to 2, so you must sell twice as many units (100% more) just to stand still.
Tip. On thin margins, a bundle or a free add-on usually costs less than a straight discount.