I think it's worth $60, I mean it has the basic features that Halo 3 had.
Halo 3: ODST = New Campaign, Multiplayer, Forge, Theater, And a BONUS, Firefight, Reach Beta Invitation. Price: $60
Halo 3 = Campaign, Multiplayer, Forge, Theater. Price: $60
Plus, Mythic maps, which would've been $20 anyways, so technically, you're paying $40 for the game, and $20 for the maps.
EDIT: Also, I love the new tactical feel to the game. you constantly have to change your weapon according to the enemy you're fighting and the most useless weapons became useful all of the sudden. Also, the new enemies like red Drones that have shields and the different Hunters and Engineers add a totally awesome twist to it. Plus, I think removing the BR was a good choice because all I always heard in Halo 3 MM was, "I'll grab that BR." "You suck at BRing!!" "I'm beast with the BR." But, let's face it, the BR was the easiest weapon to use in Halo 3. The first time I played the game I got the top of my team in Big Team and my tool of destruction was the BR. Overall, I think the expansion is GREAT.
EDIT 2: And let's not forget, the 'game' is an expansion.