a) People don't pay what they can get for free
basic principle
Search on the peer to peers networks for your "super mega rare songs" and i hope you can make a conclusion if you find anything
b) The price: if it's too cheap people don't believe it or are not confident enough to buy.
By the way, what kind of music is it ?
nowadays anyone knows how to download a mp3.
And well you ask if they prefer CDs instead of mp3... that depends on the music niche that you are promoting.
May i ask, are you selling copyrighted music? arent you scared?