The proceeds from the sale of any bonds issued under authority of this chapter shall be applied only for the purpose for which the bonds were issued: PROVIDED, That any accrued interest and premium received in any such sale shall be applied to the payment of the principal of or the interest on the bonds sold: AND PROVIDED FURTHER, That if for any reason any portion of such proceeds shall not be needed for the purpose for which the bonds were issued, then such unneeded portion of said proceeds shall be applied to the payment of the principal of or the interest on said bonds. The cost of acquiring or improving any facilities shall be deemed to include the following: The actual cost of acquiring or improving real estate for any facilities; the actual cost of construction of all or any part of the facilities which may be constructed, including architects' and engineers' fees, all expenses in connection with the authorization, sale and issuance of the bonds to finance such acquisition or improvements; and the interest on such bonds for a reasonable time prior to construction, during construction, and for a time not exceeding six months after completion of construction.