Emerald Medallion

If there are additional costs to cast a 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.

If a spell you cast has {X} in its mana cost, you choose the value of X before calculating the spell's total cost.

The ability doesn't change the mana cost or mana value of any spell. It changes only the total cost you pay.