I'm also thinking that with GH2, since the songs are copyrighted, liscensed music, if part of the high cost is going as royalties. Music that you can legally buy online, there's an agreement with the artists and their production companies so they get part of the profit.
Just an idea. (I don't have GH2 but looked at the prices for the DLC, and yeah, it's a bit extreme... but then again I have LOTS of music on my computer, some of it, I admit, I didn't pay a dime).