Refunding bonds may be issued in a principal amount sufficient to provide funds for the payment of all bonds or indebtedness to be funded or refunded thereby, and for the payment of all expenses paid or incurred in connection with the calling, redeeming, retiring, or paying of such indebtedness or outstanding bonds, and the issuance of such refunding bonds. The expenses may include the amount necessary for the payment of interest upon the indebtedness to be funded or the bonds to be refunded to the maturity or redemption date thereof, the amount necessary for the payment of interest upon the refunding bonds from the date of delivery thereof to the date upon which the principal of the outstanding bonds to be refunded will be paid whether at maturity or pursuant to a call for redemption thereof, or pursuant to agreement with the holders thereof, plus in any case the amount of the premium, if any, required to be paid in order to call or retire the bonds to be required.