Esper

If a spell you cast has {X} in its mana cost, you choose the value of X before calculating the spell’s total cost. For example, if an artifact spell’s mana cost is {X}{X}, you could choose 2 as the value of X and pay {3} to cast the spell.

The set of artifact creatures affected by the chaos ability is determined as that ability resolves. Artifact creatures you begin to control later in the turn and permanents that become artifact creatures later in the turn won’t get the bonuses.

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.

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.

The cost reduction can apply to alternative costs such as prototype costs.

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