The joint board shall keep careful account of its necessary expenses and shall apportion and charge the same to the counties joining, and certify to the board of county commissioners of each such county an itemized statement of the entire account and of the proportionate part of such expense charged to such county and the board of county commissioners of such county shall cause the same to be paid out of the general fund of the county.

County current expense fund: RCW 36.33.010.