I work for a big developer/publisher and I know more or less how much certain games currently in development cost, but I'll admit it's difficult to figure out how much is spent on marketing. At most I would expect 20,000,000 for marketing, and the actual development cost of a high profile large-scale game like GTAIV and the development of its engine can reach 40,000,000, at most 50,000,000. So let's pretend it's 50,000,000 plus 20,000,000, this means 70,000,000 total; production, marketing, shipping etc. Everything.
$50 million for some DLC is basically paying for the whole development cost, at least. It's way overblown obviously, but at the same time this absolutely guarantees that the DLC will be exclusive to the 360 and MS is not crazy, they probably asked for them to be of decent size, almost like half sequels. That's my guess anyway. And yeah expect the DLC to be available in stores too.