Section 295Q. Where the retail dealers or any part thereof in a trading area have caused to be made by an independent agency a cost survey of doing retail business in such area in accordance with generally accepted methods of accounting in which survey all retail dealers in said area have been given an opportunity to furnish to the surveyor information from their books and records with respect to their cost of doing business and the said retail dealers have thereafter been given an opportunity to adopt or reject said survey by written approval, such cost-survey shall be deemed competent evidence in proving the cost to any such retailer.