Players have a potential they are born with which decided how much they are worth along with their current quality.

The potential of players are based on if they train each day in the daily training until they are 30 years old. It can grow a little extra if you do well in the daily trainings.

 

To really make a difference in how your players value can grow you must on top give your players training camps and personal tranings. Each camp and personal traning increases the players lifetime potentials permanently and this will increase the value of the player as well.

 

Note that players are typically not worth much at age 17, but will grow a lot in value until they are around 21-23 years old.