Back when Halo 3: ODST was called Halo 3: Recon, it was announced that it wouldn't be a full priced title. But things have change, and out of E3 came word that ODST would be priced at this generations normal price point - $59.99. So what changed? Well Joystiq got down to the bottom of it.
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."
To be fair, that does sound like a pretty good value. Especially considering the previous 3 Halo map packs were priced at $10 a piece for $30 total.
Source: Joystiq.com