I'm reasonably sure the answer is no, but at Monster Payday I (or when hiring) can you sacrifice a ghost to the demon? Or not, since it's not a monster, and the icon is a monster icon and ghosts aren't monsters?
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.