28 C.F.R. § 100.12
(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.
(b) Reasonableness depends upon considerations and circumstances, including, but not limited to: