Well I love body slides my self, they are simply the best, however not many girls offer them, and some will only do thier front to your back, your lucky if you can get one who does front to front.
I have never found anyone who charges extra for a body slide if they do it and you go for FS, though with basic rub and tug it may be extra, simply because thats a nude with reverse basicly. The real challenge is finding girls who love to body slide. If you need info on a few let me know. Or if you know of any great body sliders please let me know.