Section 18. In determining cost to the retailer and cost to the wholesaler, the court shall receive and consider, as bearing on the bona fides of such cost, evidence tending to show that any person complained against under any provision of sections thirteen to twenty-one, inclusive, purchased cigarettes with respect to the sale of which complaint is made at a fictitious price or upon terms or in such a manner or under such invoices as to conceal the true cost, discounts or terms of purchase, and shall also receive and consider, as bearing on the bona fides of such cost, evidence of the normal, customary and prevailing terms and discounts in connection with other sales of a similar nature.