The prices of farm implements, machinery, and repair parts therefor, required to be paid to any retail dealer as provided in section 69-1501, shall be determined by taking one hundred percent of the net cost on farm implements, machinery, and attachments, and eighty-five percent of the current net price of repair parts therefor as shown upon the manufacturer’s, wholesaler’s or distributor’s price lists or catalogs in effect at the time such contract is canceled or discontinued.