Mizzix of the Izmagnus

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.

Experience counters are the second kind of counters a player can have, joining poison.

If an instant or sorcery 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 that spell's mana cost is {X}{R} and you have one experience counter, you could choose 5 as the value of X and pay {4}{R} to cast the spell.