Hearthstone: Curse of Naxxramas will be free for all players, the rest will cost a good chunk of money. Blizzard announced its pricing for the solo adventure pack today, stating that players will have several options to purchase Naxxramas including a $24.99 package that includes the full deal.
The first of five wings of Naxxramas will be offered for free, with the remaining four wings available to purchase separately at $6.99 apiece or together in bundles of various size. The only option to buy the wings using in-game gold (and the only potentially free path) is to buy the wings at 700 gold each.
Once Hearthstone opens the doors to Naxxramas, the devs will unlock a new wing at the rate of one per week. Blizzard also announced that it will offer heroic versions of these wings -- and new card backs -- once they're beaten in normal mode.