I don't know the real number for the economy as a whole, but the experience I do have (mostly in the higher risk/cost "card-not-present" scenarios) actual transaction costs are always higher than they seem at first glance.

For example, retailers bear the majority of fraud risk. Add that in, and the cost of anti-fraud.. it adds up. Then there's the consumer side, where users "pay" fees, pay interest rates..

My point is that all these digital payment (including the paper-originally CCs) tend to become bottlenecks and oligopolies. Banks, CCs, gateways.. all tend to centralisation, regulation and low competition. All the ingredients for price inflation.