I used to charge $45 per page (pencils-inks-tones) but that was when I was faster and not as good as I am now. Even then, that was way under charging. If I were to do it now, I would charge $90 a page for just pencils and inks, since that is the standard rate for rookie artists where I live (though that is the pricing for serialized artists and includes tone). I'm not really a rookie, but a major publisher hasn't published any of my stand-alone work under my own name, so my resume is lackluster.
It also kind of depends on the project, the length, how much time I'm given, and whether I enjoy the idea behind it or whether it's purely a "just business" thing. If I have to do character designs and thumbnails I am going to charge more. I don't really do this kind of thing anymore and haven't for a few years now since I make enough money to support myself with my own webcomic, yay. I also don't live in a very expensive place so I don't need to charge $200 a page just for pencils.