As someone who was dumb and did the mind numbing grind to 450 it is just too much.
I think something like the following would be much more respectful of a players time and could be implemented for Ash and Iron with how much time is left
10 to 199 rewards +20 power
200 to 299 rewards +10 power
300 to 399 rewards +8 power
400 to 499 rewards +5 power
500 to 550 rewards +3 power
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Eso's idea is fair... Make greens drop +2 below the Prime band, +1 in it. I'd also change Primes to +4.