$4 buys you two new maps: Tombstone and Desolation.
As promised, Microsoft and Bungie have today released the Halo 2 Blastacular Map Pack, containing two new Halo 2 multiplayer maps. For $4 (sorry you can't use MS Points) you get the new Tombstone and Desolation maps which will work for both Xbox and Xbox 360 Halo 2 players.
Tombstone is a re-imagination of the classic Halo map Hang 'Em High, will all-new graphics, features and hidden surprises. Desolation is a remake of the original Derelict map, with new graphics and effects, and improved balancing for play in Halo 2.
The new maps should be integrated into the playlists shortly so don't be concerned if they're not cropping up immediately.
As mentioned earlier, the new content is designed for the original Xbox, before the advent of Microsoft Points, so you'll have to use a credit/debit card to make the purchase.