licensing fees for music games are enormous

They will outsell the 360 and PS3 versions regardless of the price, that's why they do it.

That's a question for Apple Corp, not Nintendo.
And I doubt you will find the game at 30$ anytime soon because of musical licencing fees...

Musical games don't get price cuts because of the agreements the software developers have to sign with the music record companies.

Those companies are not ready to give their music for free....

Wii games can be priced at whatever they want. Most publishers stick within the $50 price range because they don't want to price themselves out of the market. It seems EA and Activision are not afraid of this, and since rhythm games are the top selling third party genre on the Wii, it's fairly easy to see why.

Same reason they don't stop SE from selling DS games for more then everyone else?

I doubt Harmonix would make any profit selling Rockband Beatles at 50$ anyway.
The rumor is that the contract they had to sign with Apple Corp gives a huge share of the revenue to Apple Corp...