To get these figures, I divided the Upkeep values on page 142 of the DMG by 30 to determine how much money someone would have to make per day to enter them, then assumed a whore caught a certain number of marks per day. This is how much they'd have to charge per mark to make a living at each income level.
Meager (Miserable street rat, 10 marks per day): 1 to 2 cp per mark
Poor (Street walker, 5 marks per day): 8 cp per mark
Common (Brothel whore, 4 marks per day): 3 to 4 sp per mark
Good (High-class prostitute, 1 mark per day): 8 to 9 sp per mark
Extravagant (Contractual Courtesan, one mark every other day): 13 to 14 gp per mark
Of course, you should fiddle with this however you deem necessary. You know the formula now.