I'd like to give them the benefit of the doubt, but here's an example:
For Idolm@ster 2 on PS3 (in Japan), they release new songs (yes, just a single song) at approximately $20 USD (2,000 yen).
That's not true! New songs were 'only' 1800 yen!
Also a new song in Idolmaster is a little more difficult than your average rhythm game or something since there are a bunch of different characters who have to be recorded doing every possible part of the song. Which is not to say it justifies the prices...it doesn't. Idolmaster DLC is ridiculous. I should know since I have the game...but on the other hand it's totally playable without any DLC. None of it really feels necessary. It's there for the crazy hardcore fans who can never get enough...
I think the only DLC I actually bought was the first Hatsune Miku one...because it's Hatsune Miku. Sorry, I can't help it.