It may depends also on the team played, it's Barcelona that want to buy a PSG player, with 41% of signing. It's not like he's moving to Anderlecht.
Don't care about the money system btw, it's very hard to reproduce... Even for professionnals, in estimation terms.
Creating a system of money based on Overall relative to the player's age is not impossible, but very hard.
I've checked it on real football websites how to calculate the value of a player, and estimations are based on so much parameters that it's giving you and headache to read.
...But on PES you've got age AND stats. So you could at least make a simple exponential system about the stats relative to how young or old the player is. I said that, but i don't know how to do it at all.