To clarify, since as written, you could absolutely pay the cost more than once, the reason the above answer is correct is that the current Oracle text wording of the card is:
"Whenever a nontoken creature is put into your graveyard from the battlefield, exile that card.
At the beginning of your upkeep, you may pay 4 and 2 life. If you do, return a card exiled with Purgatory to the battlefield."
Since the new version says "At the beginning of your upkeep", it is a triggered ability which will only trigger once and go onto the stack. As the ability resolves, you choose whether to pay (4) and 2 life or not. If you do, you get one of the cards back.
Mazzuno says... #1
Just once.
July 26, 2014 2:26 p.m.