The industrial commission shall fix the buying price of all things bought, and the selling price of all things sold, incidental to the operation of the association, and shall fix all charges for any and all services rendered by the association. In fixing these prices, while all services are to be rendered, as near as may be, at cost, there must be taken into consideration, in addition to other necessary costs, a reasonable charge for depreciation of all property, all overhead expenses, and a reasonable surplus, together with all amounts required for the repayment, with interest, of funds received from the state.