Handling, storing and moving cash isn't free. This isn't card processing fee vs. no previous cost.
You are right that the logistics of cash aren't free, however as it stands isn't cash simply more economical for small shops?
We need to look at more variables. Small businesses just don't operate in the same way as Tesco for example. For a small shop each and every sale is significant, having extra charges added to transactions as little as 40p is going to eat into your margins and you will feel it.
A massive chain like Tesco (which also happens to own something called Tesco Bank) can absorb a whole host of costs. They're not worried about per-transaction margins, they could get away with losing money on a thousand transactions it won't hurt and gets absorbed by the things that are making them profit, that's how companies as big as Tesco work. A small shop-keeper is thinking/worrying about his margins on each and every transaction.
The question is, are the current charges (the one that the card processors impose - not the 50p the corner shop is charging) a good deal? Is it a fair price we are paying? Is it value for money?
Because with cash, you need to employ the people who handle the cash, it's manual and there are jobs involved.
But with a card processing terminal, I think a per-transaction fee is unacceptable. There is no extra cost for authenticating a digital funds transfer on a system which is already autonomous. There's a cost to install a terminal, and to maintain the back-end systems, but the transactions/authentication itself is fully autonomous, therefore it's silly for them to get a PERCENTAGE cut of each transaction.
There should only be a small line rental fee, and I don't see why it should be anything more than £10 per month. Look at BT's line rental, they have to run a nationwide data transferring network. It's like BT charging you extra everytime you open spotify.
The actual transaction value is completely meaningless to a card processor. The EXACT same process occurs whether it's a 1 penny transaction or a £1000, so how on earth can they take a percentage cut of each transaction separately, even if it's 0.0001, it's wrong. It defies the laws of mathematics.