This isn't what the OP asked for, but something important to consider.
A courtesan, especially a supernatural one in a magical world (and a place like Sigil) obviously isn't just going to satisfy one's physical needs. A simple prostitute can do that.
No, what a courtesan of such a caliber (and especially a supernatural one) supplies not just what the client wants, but what they need. Alleviating loneliness, insecurity, shame, pain, and other shortcomings. The courtesan becomes what the client needs, such as a confidant, supportive shoulder or one who takes control for the client. She gives what they are not finding anywhere else.
She can give them what they cannot have. Relive a past glory? One more night with your (lost) family or a family you've never had? The love and adoration of millions? The success of a lifetime? Not only can this illusion be created, but it can be felt, experienced, and invoke the real emotions those fantasies have. Their heart's desire, not just that of the body.
Fulfill that, and the person will give anything.