It's rumoured the expansion will be sold in "wings" much like how the actual Naxxramas in World of Warcraft was laid out. Apparently Hearthstone currency will be used for purchasing the upgrade with the first one being free, the next will be 500g or $9, the next will be 1,000g or $18, and so on until unlocking the Frostwyrm Lair for 2,000g and $36.
This seems ludicrously overpriced and more than likely they will be sold for 500g or $9 a piece. The way Hearthpwn interprets this unlocking, all of the expansion content will cost you 5,000g or the equivalent of $90.
Past Blizzard expansion pricing has launched expansions for their games in the $40 range. 2,000g total for everything OR the equivalent of roughly $36 makes more sense than $90 and 5,000g.