Correct. Whenever you cast a spell, each extort ability of permanents you control triggers once. Each time an extort ability resolves, it allows you to pay 1 mana to drain life from your opponents. It does not allow you to pay 1 mana multiple times.
Simple questions like this are often answered by a card's Gatherer rulings. From Crypt Ghast 's:
4/15/2013: You may pay W/B a maximum of one time for each extort triggered ability. You decide whether to pay when the ability resolves.