Unfortunately, I can't tell you with precision. However, the rumour that BB-Charles likes to plant is that the differences, especially above L5, are minimal.
I have heard this rumour, but it is still not really enough to determine if level 6 trainers are overvalued. For example, if I am doing one position training only, maybe it is worth it... Or perhaps it is worth it if I am training 2 U21 prospects.
I have yet to see the evidence for or against the current price levels. Also, it depends what price you are talking about, the salary or the initial bonus. I say this because it is easy to find a price model where even a $3 million bonus makes sense, provided it made sense to keep the trainer until he had a salary of $100,000.
My own research seems to indicate that the main problem is that people don't hold on to their staff for long enough.
Run of the Mill Canadian Manager