Can Demons "eat" ghosts?
No, you can't sacrifice a ghost to a demon. From the final page (24) of the rules, under the description of ghosts:


  Ghosts can not be used to pay the cost
  for a demon.


And then later, under the description of demons:


  Heh, heh. That's me. I'm always happy
  to help you reduce your overhead. But
  don't think you can get away with
  feeding me ghosts. They just go right
  through me.