From experience, when I was an avid drawer and still had an imagination (I wanted to be a comic book illustrator), I could pump out a pencil drawing in the course of a high school or college class (around an hour). Adding very detailed clothing or armor adds another hour or so, detailed backgrounds could add 2 hours. Coloring I never got around to learning and still haven't. If I was going to go back and ink the drawing then that would add another half hour or so for the figure and an hour or so for detailed background. So that's roughly how long it took for me (20 years ago); how much it costs is completely up in the air - it costs as much as you're willing to pay and the final product that you're looking for (pencil, inked, or fully colored plus with or without backgrounds and the total size of the image - mostly they fit on a piece of paper but some people want large drawings).