Battle.net has details on the first expansion for Hearthstone and pricing details on the card-based strategy add-on (thanks GameInformer). Here's a bit:

Players who begin their adventures in Naxxramas during the launch event will gain free access to the first wing: the Arachnid Quarter. The launch event will last roughly a month or so, meaning you’ll have plenty of time to get your foot in the door—we’ll have more details to share about that later on. The web-choked corridors of the Arachnid Quarter are home to the bosses Anub’Rekhan, Grand Widow Faerlina, and Maexxna. Defeating these bosses will add new cards to your collection and give you a taste of the dangers that await you deeper inside Naxxramas.

After the Arachnid Quarter, the next four wings can be purchased for 700 gold per wing or $6.99 USD per wing. Players can earn the in-game gold needed to unlock the next wing of Naxxramas by completing various quests or emerging victorious from the Arena, or they can choose to unlock the next available Naxxramas wing using real money.

If you’re looking to swing open multiple gates at once, Curse of Naxxramas bundle packages are also available. Spiders are just so good at making things into neat little bundles. . . .