Well, I dont own a head yet, but I believe it depends on what kind of head, dreads and bio you are wearing. After reading countless of post saying that some dreads and bios are heavier then others, it sounds like the weight can vary alot. :S
I would say the absolute minimum you should expect to be carrying is 7-8lbs. I'm sort of guessing on the mask weight because mine is away being painted, but I seem to remember my half mask weighing between 2-3lbs. Bio - 2lbs, dreads - 2.5lbs, dread beads - 1lb.
You might come out a little lighter than that, or a little heavier. If you start adding fancy dread beads or other ornamentation it will go up from there. Maybe Bovine13 will chime in on how much his ceremonial suits head weighed...I think it was well into the teens.