As used in this part, unless the context otherwise requires:
less any legitimate trade discounts, but exclusive of cash discounts for prompt payment, and plus a mark-up amounting to not less than the minimum cost of distribution by the most efficient retailer, which mark-up, in the absence of proof to the contrary, shall be six percent (6%);
less any legitimate trade discounts, but exclusive of cash discounts for prompt payment;
T.C.A. § 47-25-202