I charge by overall pixel size and level of detail required. I'm working on a commission right now that's 1300x1600 pixels, with full geographic detail but no text, and I'm charging the client $45. Can't say if that's reasonable or not; this is only my fourth (paying) job, so maybe I'm waaaaay undercharging, I dunno. It seemed fair to me though, and the client had no problem with it, so I guess that's all that matters.
I don't like to gouge people, but I think the level of work you do requires an equal level of pay. It's just kind of hard to quantify what that rate should be; something that takes me an hour or two to do might take someone else two weeks. If the end product for both of us looks the same, who deserves more pay?