Duh! Me :) based on what I think the market suggests.
Fullbacks simply are not that valuable. About 5 very good ones have moved this summer already. One can gauge what a fair price is based on those prices and also the historical prices of fullbacks.
And its clear to see in the prices so far this year: 24, 14, 15, 8, and 12 million euros what a fair price might be for a very good but young fullback. 40 million pounds really isn't it.
I know the phrase "that's the market these days" is often used by fans to justify what clearly seems excessive. Many would disagree and would conflate issues. Fair enough, but for me the general prices of players around your quality often help set your price.
But then again, I could be wrong and this could be viewed as a flap :) But o doubt it.