Your players lose value because they are older and in a harder League (as represented by the stars). The younger the player the less change in value, some hardly at all. I don't know any way to calculate it, maybe someone smarter has figured that out. You could record your wages/value in a screen shot and compare it the next season. The level I am now at the values are all very close, but early on there is vast difference.

The value isn't as important as the wage to me, since that goes up when you re-up his contract, no matter they lost value. Wage in the early levels doesn't matter as much, but can be an indicator of his training quality if it is a low wage and high value.

We are in Season 63, so there could be some managers of that level (I am sure there is level 50) At those rarefied heights there is mixed level Leagues and server holds to keep parity.