By the power of truth, I, while living, have conquered the universe.
As stated from the beginning, the very announcement of Halo 3: ODST, Bungie clarified that this is a prequel. That's why it is called Halo 3: ODST, as an add-on to the same title.
Who cares about Firefight matchmaking? So long as you have a little thing called friends, you'll be able to play it and have fun.
Lastly, it's a Halo game so it will sell regardless. The renowned title that every gamer reveres as either the best or worst will have millions forking out money from their wallets. I managed to get my copy for £29 on Amazon as opposed to the usual £40 for a game. I'm not complaining.
And Bungie don't control the price, Microsoft do.