For the purposes of this article, "cost" shall be computed as follows:

(1)(A) When product is purchased by a person from an independent entity, the lowest invoice cost to the person from the independent entity for product of like grade and quality within 15 days prior to the date of resale of the product by the person; or

(B) When product is purchased by a person from a related entity, the lowest transfer price charged to the person for product of like grade and quality within 15 days prior to the date of resale of the product by the person, provided that the transfer price is reasonable; or

(C) If neither subparagraph (A) nor (B) of this paragraph applies, the lowest posted or published wholesale price of all sellers of product of like grade and quality normally serving the geographic area in which the person is located during the 15 days prior to the date of resale of the product by the person; plus

(2) A reasonable cost of doing business as determined pursuant to generally accepted accounting principles; plus

(3) Freight charges and any credit against federal or state motor fuel or sales tax not already included in the invoice cost, transfer price, or lowest posted or published wholesale price of the product; less

(4) All trade discounts, allowances, or rebates actually granted to the person on the product; provided, however, such trade discounts, allowances, or rebates are received on proportionally equal terms by all other customers competing in the distribution of such products.