Arena is running one of those deals today where you can trade in 50 coins for 550. Players also have the option of trading in 10 gems (roughly half a U.S. cent) for the same amount.
500 coins is enough, for example, to enter a Standard or Historic format Event. Or about half the cost of on 8-card pack of Standard or Historic cards.
I am not sure whether this promotion follows a pattern or what. I've seen it mentioned in memes about the game so I guess that it's fairly frequent?
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