Some sleuthing by a Redditor trawling through Grand Theft Auto V's warm, numerical entrails has revealed files pointing to something called "cash cards" of various denominations ($100,000 up to $1.25 million). There is also the following description:
Sounds like microtransactions. Given Rockstar's intentional distancing of Grand Theft Auto Online and Grand Theft Auto V, it could be part of a greater model moving forward, GTA Online was described, "as a different entity, a separate thing, and it'll grow on its own. It'll be GTA Online; it's not part of GTA V. Obviously it's set in the GTA V engine, but it's going to grow and evolve into its own thing."
Looks like that $1 billion could keep ballooning well after everyone who's interested has a copy of the game. I don't want to automatically assume this will amount to grand theft nickel and diming, but there are sensible reservations to microtransaction models.