You CAN buy card packs with real money. But you can also spend 100 in-game gold (which you earn from playing the game) for a card pack. (the gold cost per pack might be off, since I'm only recalling it from having watched people play it) You also get free cards for each class when the class you're playing as "levels up" from playing matches. And you can disenchant extra cards to get dust which can be used to straight up make any card in the game.
They also monetize the game by offering an "arena" mode where you pick from 3 random heroes, then 30 sets of 3 random cards to make a deck. Lose three times and that deck is gone forever. (you only play against other people who have done the same) You can pay 100 gold to get into that mode, or a couple dollars.
So they're being pretty smart about monetizing the game.
They also monetize the game by offering an "arena" mode where you pick from 3 random heroes, then 30 sets of 3 random cards to make a deck. Lose three times and that deck is gone forever. (you only play against other people who have done the same) You can pay 100 gold to get into that mode, or a couple dollars.
So they're being pretty smart about monetizing the game.