28 CFR 100.12: Reasonable costs.
(a) A cost is reasonable if, in its nature and amount, it does not exceed that which would be incurred by a prudent person in the conduct of competitive business. Reasonableness of specific costs must be examined with particular care in connection with the carrier or its separate divisions that may not be subject to effective competitive restraints.
(1) No presumption of reasonableness shall be attached to the incurrence of costs by a carrier.
(2) The burden of proof shall be upon the carrier to justify that such cost is reasonable under this part.
(b) Reasonableness depends upon considerations and circumstances, including, but not limited to:
(1) Whether a cost is of the type generally recognized as ordinary and necessary for the conduct of the carrier's business or the performance of this obligation; or
(2) Whether it is a generally accepted sound business practice, arm's-length bargaining or the result of Federal or State laws and/or regulations.
(c) It is the carrier's responsibility to inform the Government of any deviation from the carrier's established practices.