The bandwidth will cost less than twenty cents extra, probably far less (based on a rough estimate of S3 bandwidth costs, starting $0.12/GB down to $0.05 and under at scale). Because of peering agreements, it could actually be free. Yes, I realize there's more to it than that, but the cost difference is tiny at Apple's scale.
It also feels... scammy. "For just 25% more, you can have the version that doesn't suck!" That kind of attitude does not make me want to do business with them. Of course, this is just as much the fault of the last five years of TV marketing talking about how awful anything SD is, when in practicality it's more than adequate unless you have an absurdly large screen.
The true answer lies in price discrimination.