According to a post on Imeem.com, Mötley Crüe's 1989 album Dr. Feelgood will be available as Rock Band DLC starting Oct. 14. We've contacted Harmonix to confirm the information. There is currently no price for the album listed and we really don't have a baseline for album cost to speculate, but standard pricing has been $2 for individual tracks.
Even if Mötley Crüe is the source of the information, that doesn't necessarily mean that date will come to pass. We'll update as soon as we get more details. [Thanks mdouet]