Curse of Naxxramas also has bundle pricing: $25 for all five wings, $20 for the final four, $15 for the final three and $10 for the final two. There isn't an option to buy bundles with in-game gold, but if you play through the first wing during the free event, you only have to purchase the remaining four wings. Buying bundles doesn't unlock areas earlier than their weekly launches.
Curse of Naxxramas features a Heroic mode for each wing that unlocks after you defeat all of the area's bosses. In Heroic mode, bosses "will prove to be a challenge for even the most veteran Hearthstone players," Blizzard says. Beating all Naxxramas bosses in Heroic mode unlocks a new card back. Pretty!
There's no release date for Naxxramas, but Blizzard is saying "soon" quite a lot. Last we heard, it was due out in July.