I mean, what has playing for U21 got to do with higher price in the market?
The "businessmen" know that they can sell a player who is on an U21 team for more than the identical player who is not. It just has a strong psychological effect on potential buyers.
I think the bonuses could be modular but I think there are more modules.
Drafted player
Local Player
Playing on an U21 team
Playing on local U21 team
Drafted player playing on local U21 team
etc