  Existing fair exchange protocols usually neglect consideration of cost when
assessing their fairness. However, in an environment with non-negligible
transaction cost, e.g., public blockchains, high or unexpected transaction cost
might be an obstacle for wide-spread adoption of fair exchange protocols in
business applications. For example, as of 2021-12-17, the initialization of the
FairSwap protocol on the Ethereum blockchain requires the selling party to pay
a fee of approx. 349.20 USD per exchange. We address this issue by defining
cost fairness, which can be used to assess two-party exchange protocols
including implied transaction cost. We show that in an environment with
non-negligible transaction cost where one party has to initialize the exchange
protocol and the other party can leave the exchange at any time cost fairness
cannot be achieved.
