There are two venues that perfectly suit your needs:
Manhattan and Judy's TOO in Tongren Lu / corner of Yannan Lu. Freelancers in these bars will quote you RMB 1500 for long time. Depending on the weekday and her looks, you will most likely end up paying something between 800 and 1000 for a night of sweet love at your hotel room. There is no hassling for drinks in this bar, like in so many other places in Tongren Lu.
Pls make sure that you negotiate the service-portfolio as well as the duration of her stay upfront.