Mont. Code Ann. § 16-10-303

When a cost survey pursuant to cost accounting practices used for income tax reporting purposes has been made by the department, a trade association, an industry group, a wholesaler, or a retailer to establish either the lowest cost to the retailer or the lowest cost to the wholesaler, or both, for the state, the cost survey is competent evidence for use in proving the cost to the person complained against within the provisions of this chapter.