I "add" my own calculation.
i Had Ad Sawyers in my team, trained (but not drafted) from 18. When he was in U21 team and playing, he give me back around (approx) +2k. It was 1/33 of his salary, approx. And according to what i think, another things which influences the merch is the ranking of Nation in BB, if the player is foreign than your playing country. (IRL was nearly 85th at that time).
Instead, when he become a NT player, i receive nearly 9k, and his salary was so nearly to 95k--->1/10.
I had, instead, Manduchi, U21 Italian guy, and until he played for me and had the italian flag on the bottom of his face, i received a boost of +6/7k (his salary was 68k at the start of season), so 10% nearly.