The average payment rate for the cost report year shall not exceed the contractor's average customary charges to the general public for the services covered by the payment rate for the same time period. The department will pay public facilities rendering such services free of charge or at a nominal charge according to the methods and standards set out in this chapter. The contractor shall provide as part of the annual cost report a statement of the average charges for the cost report year for services covered by the payment rate and supporting computations and documentation. The contractor shall immediately inform the department if its payment rate does exceed customary charges for comparable services. If necessary, the department will adjust the payment rate in accordance with RCW 74.46.531.