Sapphire Medallion

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

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.

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