the value is a strong indicator of training speed as well as their age, so if you have two 18yos with identical quality but one is worth double the other, its because he trains much more quickly than the less valuable one.

Of course, if you were really keen to understand the underlying training speeds of your players, you can just run each type of training a few times for all players over the next few days and record the gains, then take averages.

Its worth noting as well that when a player reaches a new star, their training speed slows. Its not too noticeable from 3 to 4* and 4 to 5*, but apparently if you train a guy to 6* or higher, there's a really obvious decrease in their training speed.