The cost of any water or sewer or combined system and of any improvement or extension thereof, or any part of such cost, may be assessed upon property benefited thereby, based upon the benefits received, whether or not the property abuts on the improvement. If less than all of the cost is assessed, the remainder, including the principal of and interest on all bonds issued to pay the cost, shall be paid or reimbursed to the county or counties paying it from the net revenues from time to time received, in excess of the current costs of operating and maintaining the system, from the establishment and collection of charges for connection to the system and for service furnished and made available by it to any person, firm, corporation, or political subdivision or from any federal or state grant moneys, or from any combination of these receipts.