Replacing a keeper is actually pretty difficult. It's such a specialist position that as soon as you start to show interest in throwing a decent amount of cash at it, everybody knows that the rumours about your existing first choice are probably true and the price shoots through the roof for his replacement. If we quoted Chelsea a price of say £60m, you can bet your life that figure would be all over Sky Sports news within 24 hours and any keeper that we were bidding on to bring in before he went would suddenly have their price hiked up accordingly. If you try to replace after the event, the selling club already knows what you have to spend and adjusts the price accordingly, so it's a bit of a catch 22. You can play it a little cooler with outfield players, as just because you are bidding for a midfielder or a defender doesn't neccessarily mean that any rumours about a player leavung are neceasarily true. You could be just looking to strengthen your squad, rather than replacing a key player that you are set to lose.
Tottenham went out and spent money just before Gareth Bale left for Madrid. Every man and his dog knew Madrid were sniffing, but yet they still got a few players in before finally agreing to let him go. Keepers are a little different though. You would never get a Jack Butland here to be Pickford's understudy, so as soon as the enquiry goes in, Stoke know that there is some legs in the reports that Pickford is off to Chelsea for £50m+ and Butland's price becomes pretty close to that figure because they know we're desperate.