Charges for hospitalization of patients in state hospitals are to be based on the actual cost of operating such hospitals for the previous year, taking into consideration the overhead expense of operating the hospital and expense of maintenance and repair, including in both cases all salaries of supervision and management as well as material and equipment actually used or expended in operation as computed by the department: PROVIDED, That a schedule of differing hospitalization charges may be computed, including a schedule of charges for outpatient services, considering the costs of care, treatment and maintenance in accordance with the classification of mental illness, type and intensity of treatment rendered, which may vary among and within the several state hospitals. Costs of transportation shall be computed by the department.