12 CFR § 235.3 - Reasonable and proportional interchange transaction fees.
(a) In general. The amount of any interchange transaction fee that an issuer may receive or charge with respect to an electronic debit transaction shall be reasonable and proportional to the cost incurred by the issuer with respect to the electronic debit transaction.
(b) Determination of reasonable and proportional fees. An issuer complies with the requirements of paragraph (a) of this section only if each interchange transaction fee received or charged by the issuer for an electronic debit transaction is no more than the sum of -
(1) 21 cents and;
(2) 5 basis points multiplied by the value of the transaction.