The cost which has already been incurred in the past and which has nothing to do with the decision is called sunk cost. So, sunk costs are always considered to be irrelevant in a decision making process. Not all fixed costs could be sunk cost. For example, incremental fixed costs are considered to be relevant costs. So, the proposed solution of treating all fixed cost as irrelevant is not the valid solution to the pricing problem. We have to firstly find that whether the fixed costs are incremental in nature or not.