Yosef Karo, Shulchan Aruch — Yoreh Deah 336
“ If a person has medicinal ingredients and his fellowman is ill and needs them, the owner may not raise the price more than the market value. Even if there was an agreement to pay an exorbitant price because the only ingredients were in this person’s hand -he is only entitled to their market value. However if a high doctor's fee was agreed upon, the patient must pay as it is the expertise that was contracted for -and there is no market value for that. ”
