i would guess the impact of my ferda Alabora for the flag around 30k, because this is the amount my merchandising drop after he fly out of the NT.
The manager of the biggest german Center(homegrown) says he has merchandising income +100k then the average of his league, but this player alone also earns more money then his average opponents pay for their team ;)
For foreign one i have no idea, but i would say that it also depends on the salary of the player.