....less than she wants and more than the producer wants to pay
There would be a lot of variables in such a thing. It would depend on many factors: How marketable is she? Is she a big star like Vaniity or a complete unknown from the gutters of Rio? Is she a knockout or a fat over the hill clunker? What is she doing in the scene? A simple BJ or is she being gang banged by 10 monster guys? How long is the scene and is it important to the movie? and so on..
I am sure the scale ranges widely just as it does for actors/actresses in mainstream movies; some get million while others get a mere pittance.