There is a point where each unit sold stops contributing to fixed costs. If the price falls below the variable cost, selling more units increases the promotion's loss, looked at on its own.
An example with numbers
With a $40 price and a $28 variable cost, a 30% discount brings the price down to $28. At that point, no number of sales recovers the previous contribution.
Try it with your numbers
Try a 25% discount first and then 30%. Notice when it stops making sense to work out a compensating volume.
Example figures for learning purposes. They are not a recommendation and do not replace professional advice.