The point is that you're wrong; pricing doesn't reveal everything. The evidence you provided yourself: 975 RP skins sometimes have new SFX, but not all. Adding clarity through the shop by listing exactly what each skin offers helps improve expectation, since not all skins of a given price follow a uniform line.
There are 520 skins that are new models (see: Curling Veigar) or new models with SFX (see: Statue of Karthus for example, eagle cry with his dance), and even 520 skins with new particles (Leprechaun Veigar!). There're other such outliers among skin tiers, some being unfavourable (975 RP Baron Von Veigar is just a model change), which would be a pretty bad deal comparatively.
I'd be all with you if you were right, and that the pricing revealed everything accurately and you could know exactly what to expect from a skin at a given tier. Except that's simply not the League of Legends we're in, and revealing what you get for what you buy can only help us as customers.