You don't have to pay to gain access to additional cards. Each of the heroes has his or her own set of 20 basic cards (these are really only 2 sets of the same 10 cards). Defeating a hero in Practice Mode will unlock a few of the basic cards, and then the rest can be unlocked by leveling them up. Leveling up a hero is fairly simple. Just play any type of match: Practice, Casual, Ranked, Arena. Win or lose, the hero will gain XP good toward the next level advancement. Every few levels earned unlocks a new card.