Transfer fees basically represent the value of the player to the club.
If Mings was our player last year I imagine that we wouldn't entertain selling him for 20 million, judging by just how important he was.
Other clubs may mock, because to them he doesn't represent that value. It's the selling clubs prerogative now and I imagine this is why we are balking at the fees quoted for the likes of Butland because they don't represent that value to us