Looking at the transfers and bids going in this summer from premier league teams, £50m is the equivalent of £30m last summer. Absolutely insane, but look at the rejected ones for Deeney and Ighalo for a start.
For Stones, I share your concerns and, if you go back a couple of hundred pages, I said exactly the same. However, I've started reading the Pep book and I'm starting to think differently. I'm still in preseason, but on his first day, Pep identified Boateng as a player with real potential due to his comfort and ability on the ball. He therefore set about coaching him and, after a few weeks, discovered that Boateng had never been taught to defend. Not tackle etc, but to realise that everyone is supposed to work together across the defence with support from the midfield. He was just playing on instinct.
Now I look back at Stones, particularly playing in a Martinez side where no one defends and it suddenly seems much less a risk.