Inspiration gem

[A.]

Causes higher mana cost for mana reserved spells. Is it supposed to work like that ?

[Enheldor]

Yes

The way I'm understanding it from the wiki is that the mana multiplier of 115% is affecting the mana reserved but the reduction of cost the gem gives you does not affect the reservation cost.

[The_Driver]

"Mana reservation" is not considered "mana cost", hence not affected by "reduced Mana Cost". If you want to reduce mana reservation, use a skill with a mana multiplier below 1, e.g. a leveled Enlighten[pathofexile.gamepedia.com]