The reasonable cost and value of any service rendered to a city or town by a waterworks or sewerage system shall be charged against the city or town, and shall be paid when due as the service accrues from the current funds or proceeds of taxes that the city or town is authorized and required to levy in an amount sufficient for the purpose. The funds so paid shall be deemed to be a part of the revenues of the works and shall be applied only as provided in this part for the application of such revenues.