Hearthstone Mercenaries is free to play and will remain so for the foreseeable future. Blizzard-Activision is keeping Hearthstone free as a whole and only charging for certain features. The game itself is free, but not everything in the game is available without purchase or grinding.
Mercenary Cards and Mercenary Portraits can be unlocked by playing the game. As a reward for completing the 'introductionary missions,' Hearthstone Mercenaries gives players eight free Mercenaries. It really isn't necessary to acquire more mercenaries immediately since the current number of mercenaries is sufficient to play diversely.
Although the base game is free, players who don't want to grind for features have options to pay for more items. Because this game is PvE, players who pay to advance won't harm anyone playing without paying. In regular Hearthstone, bundles are available for real money and give the player more gold and mercenaries.
Hearthstone Mercenaries use mercenary coins as their currency. Players are able to obtain coins through purchases, but they must be careful. Mercenary coins are unique to each mercenary. Thus, if a player has Sylvanas' mercenary coins, they cannot use them to empower Jaina.
Thus, buying them at all is a gamble, but feel free to use cash if you have it to spare!
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