The commission shall fix such rentals, fees, or charges for the use of parking places under its control as will produce revenue, when added to the expected revenues from on-street parking meters pledged to any of the purposes specified in this part, sufficient to pay the principal and interest on the bonds as they fall due, and also to provide any additional sums which may be required under the provisions of the ordinance, resolution or indenture providing for the issuance of the bonds.