Have they asked you to do the design? If not, and you are just making an offer to get some work under your belt, then maybe free isn't a bad idea. If yes, I would charge a reasonable amount for the time spent on the design.
Will the band be using your design on more than the CD? Merchandise, website, ... etc? Take that into consideration as well and charge accordingly.
If you do the design, will you post it here? Would like to see it!
