The proceeds from the sale of any water revenue bonds issued under authority of this 1959 act N.M. Stat. Ann. § 72-4-2 to 72-4-12 shall be applied only for the purpose for which the bonds were issued; provided, however, that any accrued interest and premiums received in any such sale shall be applied to the payment of the principal of or the interest on the water revenue 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 water revenue bonds were issued, then such balance of said proceeds shall be applied to the payment of the principal of or the interest on said water revenue bonds and provided further, that any portion of the proceeds from the sale of the water revenue bonds or any accrued interest and premium received in any such sale, may, in the event the money will not be needed, or cannot be effectively used to the advantage of the county for the purposes herein provided, be invested in short term, interest-bearing securities if such investment will not interfere with the use of such funds for the primary purpose as herein provided. The cost of acquiring any system shall be deemed to include the following: the actual cost of the construction of any part of a system which may be constructed, including attorney’s and engineer’s fees; the purchase price of any part of a system that may be acquired by purchase or condemnation; and the interest on such bonds for a reasonable time prior to construction, during construction, and for not exceeding six months after completion of construction.