The value is dependent on their ability, your team ability and
league that it is in etc.
For example, if you have the same Cristiano Ronaldo in Man U and in
Crewe Alexandra, his value at Man U will be higher. 
AFAIK, it's not really controllable. You might want to give him a
bumper contract and see if it increases the value. Of course, if
the player's in the last year of his contract, his value will go
down. That applies also when the player's past his peak years.
