That depends, if you already own Halo 3, it's more of an expansion, however if you don't own Halo 3, then it's like buying a full game.
You see the thing is Halo ODST is only the same with Halo 3 in the sense that they both have the same multiplayer. The main difference in the two games is the campaign (and ODST's Firefight mode). So if you already own Halo 3, then you're basically just paying for the new campaign and Firefight, if you don't own Halo 3, then you're getting all new stuff.