Well, DLC doesn't require packaging, distribution, or shelf space. If your game has a way to promote DLC already built in, you don't need much marketing buget either.
And, if you use existing graphic assets, and some of your DLC files (music) were likely already made at the time of the original game publishing, 2% is all cake--pure profit directly to the publisher without any middlemen. Makes sense for the publisher to try and push as much DLC as they can, especially on projects for the music genre, especially games with a smaller or singular artist set, because it is easier to license a single artists catalog, and then pick and choose what you want to release, as opposed to negotiating rights with multiple muscians for individual tracks. I think this is why EA and MTV have been examining the evolution to more singular artist centric releases as opposed to the Guitar Hero ala carte model--its probably a lot easier to work out the licensing deal.
Yeah, the Wii version was way over priced, which I think is just too typical of publishers trying to milk as much money from the Wii as possible. Again, with a huge install base, low sales still makes you a shit ton of money.