Standard Prices for a single character are as follows:
A Sketch is 10$
Inked picture is 25$
Colored picture is 40$
Transformation / Comic Page is 80$ ((Uncolored)), Color is Dependent on Comic
Each additional character varies depending on what you're asking
Update: shading a comic page now adds +10 to the price
Backgrounds are variable, if you just want some simple object for them to sit on that can be free, complex backgrounds are out of my element so pricing would depend on what you want.
Contact me through a note if you want to commission me, I take a "do picture first, get money after" approach to avoid procrastinating.