The reimbursement of the expenses mentioned in 2° of article R. 444-3 may be either a flat-rate amount or the actual cost of the expenditure incurred by the professional for the provision of a service. Where this reimbursement is flat-rate, the amount of the flat-rate is set by the joint order referred to in article L. 444-3, on the basis of an average assessment or an appropriate reference value, depending on the nature of the expenses.