Okay i recent tested this out in my current save, and i had a **** player, used fmrte and toggle the stats using the 'make wonderkid' option. then the stats went up, and got a pa of 197 right.
so i go to game and look at him, and the report say like it says, quality player of the future. he is still 16 and i didn't sign him on a pre-contract, so i right then i went to give him an contract, and the strikes back with a 20k p/w demand and all those stuffs. (H)
the other thing make the player demand high wages above the obvious, is that he clear doesn't want to join u, simple

im my second test, i did that, and the 19 yr old LB, who has been playing XI for like the whole time at Roma, demands 70k.

so i withdrew and talk to him to the press, and he is delighted. then i make my second move, and it goes down to 40k atleast, which seems fair since he was Roma first choice LB for a long time.
therefore, some causes for a player to demand such wages are :
1. their PA (-10 or -9 ; 190 to 200 )
2. their willingness to play for his own club atm (especially if the player is playing at his boyhood club or rivals teams)
3. their playing time at their current club (XI important member)
4. club stature (if ur a sugar daddy team, it no surprise really a incoming player or current player will go rampant on their wage demands)