Bale - £85m
James Rodriguez - £63m
Angel Di Maria - £59.7m
De Bruyne at £57m or lower would be a great transfer. His last season was every bit as impressive as the one that got Bale his move, and he's the same age. He's done more than James Rodriguez and is a year older, but maybe doesn't have the marketability.
His age means a lot as well. Unlike an older signing, he'll still have a high resale value if things go bad, it means if things go well we get to keep him for 6 or 7 years, it means he can still improve.
Also, for people who wanted Pogba as a statement signing, De Bruyne is turning into one as well. Wolfsburg determined not to sell, Bayern and the whole of Germany seemingly trying to make him stay so they can get him later... signing him now would be a statement.
Not to mention there's not really anyone else even remotely available who is a better player or a better fit.