105 ILCS 5/12-22
The tuition paid shall in no case exceed the per capita cost of maintaining the high school attended, which tuition shall be computed by dividing the total cost of conducting and maintaining the high school by the average number of pupils enrolled, including tuition pupils. Depreciation on the building and equipment of the high school attended shall be included as part of the cost of maintaining the high school attended, and the amount of annual depreciation on such building and equipment shall be dependent upon the useful life of such property. The board of education of any non-high school district may audit the claims of any school submitting a claim for tuition, and shall, after making request of the school board and the school treasurer, have access to the school records and financial records of the district for the purpose of making the audit.
The school board of the high school that the tuition pupils attend shall certify not later than August 1, of each year, to the non-high school board, the estimated amount of the tuition charges for the succeeding school year.
105 ILCS 5/12-22