I don’t know the rates in the US, but in Europe the cost of cash and credit card are roughly the same, at least when your average sale is around 5 € or so. It’s probably more expensive for the merchant in the us, otherwise credit card companies couldn’t offer these insane cash back incentives.
Illegal to charge more: true in the EU, but we capped the interchange fees to 0.3% for credit and 0.2% to debit, which is the part the credit card companies and banks get. A payment provider will add his fees to that, but those don’t are facto monopolies like MasterCard and VISA and their issuers hold.
But their contracts with vendors already stipulated a “can’t charge more” clauses. As far as I’m aware they never got successfully challenged in courts.
