The value of a player is the value of all expected future contributions to his teams. In that sense, Xavi is not worth THAT much simply because he does not have THAT many future years left to contribute in. Only in that sense is it justifiable to value players such as Nigel de Jong higher than Xavi. You COULD argue that 2-4 more years of Xavi is worth the same as 7-9 more years of Nigel de Jong. I don't agree with that, but it's arguable.
What is NOT arguable is to say that Gareth Barry is worth more than Xavi. Barry is 1 year younger than Xavi, and by far a lesser player.
EDIT: Another one I like is David Silva being worth more than Iniesta. They play a similar type of game and Silva is quite good, and two years younger than Iniesta, but, Iniesta is definitely a or two class ahead.
Some other laughers:
- Joleon Lescott worth the same as Pique, despite being 5 years older and just a way worse player.
- Balotelli worth more than Pedro
- Maxwell worth more than half the Barcelona team, including Xavi.
I could go on.