The transfer market in England is bonkers. Swansea have rejected a 40m pounds bid for their player. So if you think about Walker transfer this way, it's understandable. Also, I think ManCity are now FORCED to sign English players, they have almost none in their squad.
Otherwise, it's hard to understand the fee paid. They really, really overpaid.
I can only sympathize with Guardiola. Barca have been looking for a right back, and there aren't many good ones available. Bellerin, who seems like a limited runner, would cost more than 60-70m. And Arsenal don't even want to sell him, he'd be sold only if he forces his way out. The alternative we found is an unproven kid from Benfica. This is not a question of money, there just aren't enough top quality players available for all.