The commission, in determining the rates to be charged by any utility under its jurisdiction, shall ascertain whether an advanced or fictitious cost price, or a price in excess of the fair market value of any commodity, machinery, equipment, material, or service has been paid or is being paid or charged, by the public utility. If it shall appear that any such fictitious or advanced price has been or is being paid or charged, the commission shall fix and allow as a part of the valuation or rate basis only the reasonable and fair market price of such items, at the time of the purchase, eliminating all such fictitious or excessive prices or values.