Seems like there's a whole lot of confusion behind why they're making this mana cost change, so I'm cross posting this from a reply I made on the the MMO-C Arcane thread.
With the latest Savant hotfix and current Mastery stacking, it is possible to reach the 65-70% levels of Mastery. This converts to a damage bonus of 90-95% per Arcane Charge. This is dangerously close to the 100% mana increase per stack. In other words, Arcane Charges is starting to fail at making spells more expensive per damage dealt. This will get worse as we enter Nighthold with higher itemlevel and slight increase in secondaries.
If we continued on this path, any Mastery buff effect or trinket would make it so that higher Arcane Charge spells are more mana efficient than 0 Arcane Charges. Once that happens, the ideal DPS rotation will involve standing around and waiting to cast AB4/AE4 instead of resetting stacks with ABar. That's a disaster.
To create some room for rotational design, they increased the mana cost. Going from 100% to 125% mana increase per Arcane Charge easily adds room for another 50% worth of further Mastery gains. The issue of "it's a nerf" is pretty easy to fix - just change the base coefficient so that Arcane Blast overall does more damage at every level.
What we should do now is obvious: test things, do some math, fix some spreadsheets and sims so we can confirm how much of a nerf or a buff this change is. Then, tweet these numbers to devs, so that they can use it to adjust the base coefficients, such that it ends up being what Celestalon said it should be in the first place: a significant buff.