For anyone who's played Grand Theft Auto Online, this really shouldn't come as any surprise.
Almost a year after launch, the online community for the game is still as vibrant as ever and now that most of the rampant cheating / hacking has been weeded out, Rockstar is starting to rake in the cash with DLC and microtransactions. According to documents filed regarding a lawsuit taken by one of Rockstar North's top producers, "GTA Online has the potential to achieve the greatest profit margin of any game created in the GTA franchise" with the microtransactions netting Rockstar almost 100% profit margin.
So far, Grand Theft Auto V has sold over 60 million units since its release and continues to sell well on online stores, with new gameplay modes and DLC keeping the experience fresh for players. Microtransactions, of course, are part and parcel of any game nowadays, but the level of money being drawn in by GTA Online is pretty astounding. Overall, the documents filed in the lawsuit claim that Grand Theft Auto V has made Rockstar a total of close to $3 billion so far.
Considering how well multiplayer DLC has worked for GTA Online and Rockstar, it's no wonder that singleplayer DLC has long been mooted for the game.