Psychics and Energy Workers disagree on this very topic, many won't charge and take "love" offerings, bliss donations, or barter services. Then again, many folks do charge for these services. I personally don't think Spirit/God/Goddess will torch my Karma for charging a small fee for my psychic services or energy work, since I need to feed the physical body and have two girls to care for. I also don't charge folks whom I know have little cash.
So what about charging for investigations, travel expenses, etc...I think the Warren's only charged for travel away from their home state.