The superintendent of schools of the county may employ such extra help as is necessary to perform the labor for the maintenance and repair work, as well as to provide for the supervision and transportation of the labor together with the equipment and materials for the work. The cost price of the maintenance and repair services to any school district is the original cost thereof and in addition a sum sufficient to reimburse the county superintendent of schools for all supervision, transportation, equipment, and other expenses, but the sum added shall not in any case exceed 10 percent of the cost of labor and supplies.