We contacted Bungie and Microsoft for comment today, because when pricing was revealed at E3, we didn't get an explanation for the full retail approach. We've noted that retailers are pricing the game at the standard $59.99 price point, the $99 Gamestop exclusive premium pack being the lone exception.
A Microsoft spokesperson responded to us and said, "At the time of those statements, the overall scope of Halo 3: ODST was not yet finalized, and since then the project has grown increasingly more ambitious. We believe this standalone experience is much more than just an expansion. Halo 3: ODST provides a new campaign from the point of view of an entirely new character. Combine that with three new multiplayer maps, the entirely new cooperative mode called Firefight, and the complete Halo 3 multiplayer collection on a standalone disc, we feel this is a good value and tremendous addition to the Halo franchise."
Chances are if you're a hardcore Halo player, you already own the multiplayer maps, so you'll basically be paying for content you already own. It still feels a bit light to us, but we'll have to play the game when it comes out in September to see if it's worth every penny.