Ruby Medallion

If a spell you cast has X generic mana 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 cost reduction can apply to alternative costs such as flashback costs.