I understand that Eternals aren't free. Even thought it's a bit disappointing, I can see that it is a purely bonus/cosmetic thing that took some time to develop.
But 850 RP per champ and per set is just not worth it. Most eternals are really basics, mostly summing endgame stats like kills and stuns, and even unique eternals are mostly "ennemies hit by your ult". Nothing so exciting really. So the work to make every champ different probably wasn't that high.
I would much rather buy a skin for that price.
I think 400 Rp is the most it should cost. Maybe at 500 I would buy it for my main, but not for more, that's just not worth it.
Maybe you can do some promotion, like half price if you have mastery 7 ? It makes sense to have eternals mostly on high mastery champs.
I hope you diminish the price somehow before release, else i won't buy one even thought it would be cool to be able to progress soehow once mastery 7 is reached.