It's a number set by experts according to statistics and reputation of the player and league etc. which should show the players estimated value on the transfer market. Of course it isn't something like an overall rating like in FIFA, because you can't valuate a human in terms of money, but if there's a difference of €10m it is a good way to rate a player. Everybody knows that it's hard to negotiate with Portuguese sites and Matic leaving for that sum is something completely different and unrealistic. Please read my post again. The only thing I wanted to express is that Matic is still rated lower objectively than Witsel despite playing a great season at Benfica and that getting Witsel would not be any easier, because Fernando pointed out that it could be.