The value of a player is subjective and based on more than just current level of performance, it also has to do with age, contract situation, wage, importance to the parent club, in addition to how much demand there is for him in the market and the financial capabilities of suitors. In this case, Southampton is under no obligation to sell save at the wish of the player, they will have a parachute payment for relegation, he has 4 years left on his deal, is only 19 year old and has a buy back clause for City for that amount, as well as a sell on clause (so Southampton would already make less money selling him for £40m as opposed to selling him back), and due to his age his wage demand will be reasonable. It’s all well and good if in your opinion he’s not worth that but the reality of the situation is you won’t be picking him up for cheaper in this market. Fred was £47m in 2018, Ben White was £50m 2 years ago, those figures just don’t get you players of the same calibre years ago.