Well, this "doubt" has come to an end.
Apparently, even if in the PvP stat page it's written "MP Cost Reduction: 35%", some kind of normalization occurs.
Playing with passives and attributes that further decrease the MP Cost lured me away from the correct answer...
I tried casting Queen's Throne (Base cost 180 MP), which doesn't have any kind of MP Cost reduction from selectable attributes or passives, it turns out that...
- In Dungeon I can cast it for 117 MP and it will use all those 117 (35% of 180 is 63, 180 - 63 = 117)
- In PvP it will ask me for 147~ MP and will use them all even in this case (18% of 180 is 32.4, 180 - 32.4 = 147.6)
Long story short, that 35% is actually an 18% MP Cost Reduction in PvP, while in Dungeon it would work as intended.
It's clear to me, this post can be closed.