I think no problem with my previous reply, its about the 100k++ for staff instead of a player.
Funding 100K for a staff instead of a player is actually a short term thing.
If you look at it long term, if the rate of skills increasing is increased then it is actually faster for players to reach $100K salary. So in the end (after 3-4 seasons), you will not only need to fund a 100K staff but also a 100K player.
And if we want to aim for the stars, target should be 200K for the players. So ultimately it will be 200K for player and 100K for trainer if using your strategy.
Ultimately it depends on the objective. If the objective is only to improve the U21 team, then yes, having the highest level trainer will definitely have a huge impact.
But if the objective is to improve the Senior NT team, then having the level of trainer above level 5 is not as huge impact as there is no age limit for NT so you just need to train extra few weeks with Superior trainer compared to training with a World Renown trainer.