In fairness, prices in this Quest are generally higher because of how much EXP you get.
Additionally, this Ability's main quality is being a defensive boost for you; which is understandably more valuable to a DPS like Cerys.
That aside, keep in mind that growing your MM this other way is basically free while increasing damage or defense the normal way is pricey. 850xp is about five levels in any Spell or Infinity Edge, but can sustain you much further in the long run.
I am more amused by the fact you find 850 for that overpriced, but have nothing to say about the other Shop Items.
I know that stuff is more expensive here. I took that into account.
I meant it seems overpriced relative to the other shop items. For example, adding Juggernaut to a big, powerful AOE spell is a huge change that greatly amplifies how good it is at its job, and adding what seems to be an anti-god ability to it is a huge deal and is the kind of thing that should be super expensive and require a high level as a prereq.
The Ignition Aura upgrade I can't comment on because I can never remember how it scales. How 10% of 638 relates to 1340.
And obviously I can't comment on the things with unknown prices
As for using Magic Surge defensively, that's primarily useful only if either of the following is the case.
- Daiyu isn't around to tank and we have far more APT than our opponent has dangerous actions per turn.
- Daiyu couldn't block an attack aimed at us.
Sure we can burn some actions to massively increase our defense once, but that doesn't actually accomplish anything unless we can do it as often as the opponent can use the attack that forces us to do it while still having some actions left to retaliate.
It's kinda comparable to a low end Skilled/Dodge variant in that respect. More flexible than both, but also weaker than both.
As for actual numbers on defense, right now 850 EXP could give us the ability to burn actions to add 638 defense per action. Against an opponent with two APT that's equivalent to either having 3 actions plus an extra 638 defense or having one action plus an extra 1276 defense.
To contrast with that, 750 EXP dumped into Plate Armor would add 700 extra Base Resilience while keeping 5 actions per turn. Having the DPS spend that much on defense isn't wise, so the fact that it gives a bigger benefit than the new ability does should say a lot.
Of course, getting buffed by Daiyu's (and Una's, once she unlocks it) Knight's Aria does improve the effect of Magic Surge substantially, but even then I'd still generally prefer Skilled or Skillful Dodge defensively.