Esper

That ability can’t reduce the amount of colored mana players pay for a spell. It reduces only the generic component of that cost.

If there are additional costs to cast an artifact spell, or if the cost to cast a spell is increased by an effect (such as the one created by Thalia, Guardian of Thraben’s ability), apply those increases before applying cost reductions.

Esper’s first ability doesn’t change the mana cost or mana value of any artifact spell. It changes only the total cost players pay.