I was going to chime in with my 2 cents, but "jim914109" said it all pretty well, the same way I would have. In my mind there is no real set rate. If you like out in the 'boon docks' you may find yourself able to get a model for less than that though. For example, if she was not wiling to sign a full model release I would say you could pay her less since the photo would only legally be yours to have, but not to sell/distribute/or otherwise broadcast. If your doing a test shoot that is not too far fetched.
+1 on the 'know their limits' bit. If someone is posing nude, it is important to figure out what they are okay with up front. Erotic 'spread leg' shots, Penthouse style parting of her lips
, or more. Most of the time it's best to just let it happen, don't demand they spread anything if they never said they were willing to ahead of time. Let's just say a colleague of mine more than once has had the MILF soccer mom type in a semi-nude/nude session, where the model got excited enough to rub one out while posing
, spread lips and all. He just stood there and took the pictures.
I'll say this much, I practically would insist on an escort to come with the model, first and foremost for legal reasons (modeling release witness and overall witness in general for the model's comfort. The second thing with the escort is the right personality to help out is invaluable. With the girl I shot, her husband was the one egging her on to really get explicit. If he was not there, the first shoot would have been a tad more tame on that point.
Thanks to all the posters for their input.