While on the "Strictly Business" quest, where you enslave people from around the Wasteland, occasionally Susan Lancaster will disappear from Tenpenny Tower. After hitting her with the Mesmetron and being caught reload a saved game and try again. She may be nowhere to be found. The quest marker on my map will state you have to go to Paradise Falls. After picking Eulogy Jones' pocket for the "Box Key", gain access to the slave pens. You can find Susan Lancaster inside the adult slave barracks without a collar. Zap her with the Mesmetron and collect the payment.