Pay varies by state, country, and model.
Someone posted a good ballpark post a few months back. Do a search for it. It broke down some rough guesstimates for all the different scenes, and approx. rate.
Some pay a lot more than I could ever see in the ROI. Then others low ball. But I guess in the end it comes down to, what the girl will be willing to accept, and what you can pay for them.
