Achievements imply a sense of "achieving" something. Getting the Hayabusa armor in Halo 3 was an achievement, as it required getting a variety of things done with a varied skill set in multiple modes.
Eternals are not achievements. Eternals are mere stat-tracking. Now, with that out of the way, let's look at how comparative games handle stat tracking:
In DOTA2, stat-tracking is related to an item called "Inscribed ". Sometimes these are character specific while other times these are general such as "tracked kills". These run fairly cheap at about 8 cents for an inscribed kill.
In CS:GO they're slightly more expensive as they're tied to the weapon but unless you're aiming for a knife which racks up over a hundred dollars or more, they're around 1-2 dollars.
Eternals will cost around 850 RP. Which is about 9-ish dollars? So for 3 eternals you're paying three each. On paper, this doesn't seem _SO_ bad. They're more specialized, a little more expensive, blah blah whatever.
##The problem is that this was Riot's chance (And still IS their chance) to be better than competition.
The price isn't the problem. Hell if this was in DOTA I doubt anyone would care. The problem is that Riot can do better. If this was free it'd be an awesome system. Everyone would love to track stats. Hell, even if it was cheap it wouldn't be a bad system. Make it buyable for Blue essence. Who gives a shit. The main problem is as it is now, it's a bad system.
This is Riot's chance to _actually_ be better than their competition. Oh, Valve charging you for stat tracking? Have it for free. No questions asked. If you want to earn money, there are better ways to do it like announcers, map skins and more. The fact is that it's not about the system. The system itself is fine. The fact is that it being paid is terrible. You can do better.