Nasri is only worth that on his profile because thats what Man City paid Arsenal for him, before the season so that gets updated on FM.
Spurs paid Dinamo Zagreb 14.25m for Modric.
That's why Ronaldo is valued at 80m, that's Madrid to buy him from United.
When you start playing the game, the sale value gets altered.
In truth, These players can be worth much more or much less.
Totti is worth 3m on his profile, even though, his traits are better than Lopez (who's worth 19m) Totti has never transferred to a club. There's age and other factors but you get the idea.
You obviously value Nasri more than he's worth, so its okay, if you want to keep him