I don't think you can boil it down to HD visuals = more money. It's more complex than that. If you look at HD games, when compared to the preceding generation...
1) The game worlds are larger
2) The character models are more detailed
3) The games themselves are more feature-rich (single-player and multi-player, online, etc.)
What this means is that you need to hire more front-line people (artists, designers, modelers, testers, etc.) to do all the work that's required in, say, a two-year period of time. I don't think that the quality of the visuals themselves are really that big a deal, but it's more the scope of these projects and the timelines that they're on.