ODST does come with a multiplayer disc for Halo 3 that includes all of the multiplayer DLC map packs. I believe that's a 25 to 30 dollar value just for the cost of the map packs.
As for ODST's campaign, I thought it was really good. If you can get this game for less than $30, then I'd say yes. Chances are, you can get it slightly used for much less than that.