You can't look at Mbappe now and say it is normal that he gets 40m before tax. When PSG loaned him with obligation to buy, the following contract will have been already agreed. At that point, Mbappe just had 1 season at Monaco and no world cup exploits. His status was the same as Ousmane Dembele, who looked like an equally promising talent at Dortmund. Dembele went to Barca for 105m € plus 40m € in very easily achievable add-ons (5m € for every 25 games until he has 100 games at Barca is just one of those). Der Spiegel revealed that Dembele earns 12m before tax at Barcelona (up to 20m depending on add-ons). They are not doing guess work like all the other media outlets, they are part of Football Leaks and actually have a look into the contracts / copy of contracts of the stuff they reveal. So you have two talents with the same quality, same nationality, same age, same status, both going from selling clubs to super clubs and one is supposed to earn 12m before tax and the other one gets 40m before tax? It is simply not plausible.