It highly depends on the target skill set. As an extreme case, even with lvl7 trainer you cannot cap perennial allstar if you only train OD.
For more realistic example, my players in utopia only needed 3-4 seasons to reach allstar level cap with lvl3 trainer and 2 position training, plus a few team training. This includes those with penalty (passing, pressure, ball handling at PG/SG) and those without (one on one/jump shot). They are near the limit for perennial allstar, which are their actual potential.
I agree that for higher leagues lvl4 is best value for money. Weekly salary of lvl4 is around 20k per week, but spikes to 50k per week at lvl5. Trainers with lvl5 - 7 are usually for NT or very competitive leagues, where the 3% boost in skills can change the outcome of the game. For lower leagues lvl3 also has good value costing around 10k per week.