Refunding bonds may be issued in a principal amount sufficient to provide funds for the payment of all revenue bonds to be refunded thereby and in addition to the payment of all expenses incident to the calling, retiring, or paying of such outstanding revenue bonds and the issuance of such refunding bonds. Such expenses include any amount necessary to be made available for the payment of interest upon such refunding bonds from the date of sale thereof to the date of payment of the revenue bonds to be refunded, or to the date upon which the revenue bonds to be refunded will be paid pursuant to the call thereof or agreement with the holders thereof, and the premium, if any, necessary to be paid in order to call or retire the outstanding revenue bonds and the interest accruing thereon to the date of the call or retirement.