Well, for one, it's difficult to gauge prices... You have to test it out and find what you're most comfortable with and what the audience you're aiming for is most comfortable with. You also want to provide only just enough options. For example, I offer sketches ($20), inks ($30), markers ($40), and computer ($70). You'll also want to charge extra for things like backgrounds and extra characters (other wise people have a habit of taking advantage of you and trying to get as much as they can cram into the image D: ).
Set things up the way that works the best for your self... although you'll likely change things around after seeing how it all works out. ^.^
It might be difficult to get people to bite, or it could be really easy. It really just depends on their situations at the time.