What is normal for me might not be normal for you... The important thing is to be honest and open regarding what you would like to do, and respectful of the model as a person. Then discuss what you want from the session and what you have to offer.
I offer 1/2 of the profits of any image of their's I sell. But I do that no matter what--nude or not. I think the majority of models who pose nude for me do so because they are interested in the concept I'm doing. And I always provide them with copies of all the images from a session that I end up retouching.