I am interested in producing CD compilations or providing download
capability for roughly 50-75 songs and selling the CD
compilations/individual songs through a website. How can I do this --
legally -- and what would it cost me?  The music would not be original
- it would be established, copyrighted stuff (I'm assuming).  I'm
thinking: ITunes, but on a much, much smaller scale specific to the
business I'm envisioning.

This should be helpful:

http://www.riaa.com/issues/licensing/howto.asp