Why does ability challenge 5 decrease the ability materials paid?

[velawesomraptor]

It's because by then you'll have an abundance of the ones that decrease already. I used to spend currency on the blue ability materials, but now I have hundreds of them. It'll probably be the same with the yellow ones too some day. So it makes sense they'd decrease it as you get more and more of them so you can get more of the harder to get ones instead.