Pot of Extravagance allows you to banish cards from your Extra Deck to draw cards.
However, some archetypes, like "Monarch", "True Draco" or the "Lair of Darkness" structure deck doesn't need an Extra Deck to beat an opponent.
If you use one of these types of decks, could you just put "useless" cards in your Extra Deck that you don't plan on using so there is no cost to using Pot of Extravagance?