Cal. Penal Code § 4210

The delegates from each county may enter into an agreement with the other counties for and on behalf of the county appointing them, binding said counties to the joint enterprise provided for in this article and apportioning the cost of establishing and maintaining a road camp or camps, such cost to be apportioned on the basis of the population of the respective counties as determined by the official declaration of the State Legislature determining the population of counties next preceding such apportionment.