Lets be honest, john stones is a 15-20 mil player. he is not the finished article, he is good. people saying he is 50 mil need to have a word with themselves. if we could get 30 mil for him but at the beginning of a transfer window, that would be an outstanding bit of business.
I've heard variants of this from the few people who want to sell Stones.
It's entirely irrelevant what his "honest true value" is in terms of how much we should accept.
Chelsea obviously would have bid again if we didn't have the BK/RM statements and it would have been higher than 30m. So he's already worth more than 30m according to the market. Taking 30m now -- even at the start of a window -- would be accepting less than we could receive.
Now lots of things can change but assuming he has a good season there is no reason not to think a number in the 40's is extremely likely and 50+ at a stretch is attainable. Whether he's "worth" that much is completely irrelevant. Even if we knew some deep secret which meant his real value was 10m; if we could get 50m by holding out then we should hold out.
Someone posted how Martial was "rated" at 16m or something but Man U paid more than double that. True value and market value can often be way off.
Edit: Writes long reply, gets distracted by something else, finishes it hours later then notices a bunch of other people already made the same point.