The net income from the building, facility or improvement so erected, purchased or made shall be deemed to be the difference between the gross income derived from rentals, meals, charges for services and all other revenues from said buildings, improvements or facilities, less the reasonable operating and maintenance expenses thereof. The reasonable operating and maintenance expenses shall be deemed to include all costs of heating and lighting said buildings, improvements or facilities, insurance, the actual cost of services of employees operating and maintaining said buildings, improvements or facilities, costs of food, repairs, costs of reasonable replacements of equipment and any other incidental costs not herein specifically enumerated, but which are reasonably necessary in the operation and maintenance of said buildings, facilities, improvements and equipment.