Idaho Code § 31-4908
Any costs of the district for the ensuing year, in excess of the fees and other revenues anticipated to be received directly by the district, shall be allocated to the participating counties in accordance with the annual budget approved by the district board. Costs shall be allocated on a per capita, weight, or volume basis, or any combination thereof designed to achieve an equitable distribution of costs in relation to the benefits derived to each county. Any participating county may finance its share of such costs in accordance with section 31-4404, Idaho Code. It shall be the duty of each participating county to remit its apportioned amount to the district at such time or times as specified in the annual budget approved by the district board.
Idaho Code § 31-4908