They might do it the same way they've done it in Forza 4.
They announced that they'll do DLC packs, 1 a month for 12 months, they'll always be 520MS points (so around 10 USD) each.
They then released a season pass, which lets you get all that DLC but as one off payment and you get when its released. The cost was about 2 thirds of the total for all the DLC. So if you intended to buy it all, like myself, it works out considerably cheaper to get the season pass.