55 ILCS 5/3-9012
A State's attorney who serves 2 or more counties shall receive such compensation from the State Treasury as is provided by law for the State's attorney of a single county. The State's Attorney shall be paid by the counties such compensation as may be agreed upon by the county boards within the salary range prescribed by law applicable to a single county with a population equal to the combined population of the counties the State's Attorney serves. Unless the county boards agree upon a lesser amount, the State's Attorney shall be paid the highest permissible salary within such range. The amount to be paid by the counties shall be apportioned among them on the basis of their population. Seventy-five percent (75%) of the amount provided by law to be paid from the State treasury for the services of the State's attorney in the case of a single county is payable to each of the counties served by the same State's attorney, except that the amounts paid to those counties under this Section in any year may not exceed, in the aggregate, the annual salary paid to that State's attorney from both county and State funds, in which case reduction of the State's contribution to each county shall be reduced proportionately according to population of each participating county.
55 ILCS 5/3-9012