Mich. Comp. Laws § 720.656

Sec. 6. Capital outlay and construction costs of facilities of such regional facility shall be borne by the counties comprising the region in the following manner: (a) Fifty percent of the estimated cost shall be borne by apportioning among the participating counties a percentage of the cost according to the proportion of the county's state equalized valuation bears to the total state equalized valuation of the district. (b) Fifty percent of the estimated cost shall be apportioned among the participating counties according to the proportion the individual county's school census bears to the total school census of the participating counties.