Disagree to a certain extent with the other answers. I do agree that you probably can't make a living doing just this, although it can be a profitable part of a handyman service.
The reason drywall repair is a valuable service is that the bigger companies usually won't touch little jobs. So there is a niche there for the little guy with no overhead of an office or secretaries to answer phones.
Also, most homeowners do NOT want to or know how to repair their own drywall.
Generally speaking, you have 2 things to do: repair the drywall, and paint. This can be accomplished in 2 trips, but to the best of my knowledge, no less than that.
I have 2 basic prices - one for the customer already has the paint leftover from when the wall was painted, and two for when I have to match the paint.
Day 1: repair the drywall, take a paint sample from the wall (cut out a square of paper from the drywall.) Do your first coat of (for example) 20 minute setting compound, second coat of setting compound, and finally a third coat of regular premixed compound. Patch over the wall sample too, of course. When done, go buy the matching paint.
Day 2: sand. If necessarry, touch up with a little more compound if any is needed. Use a hair dryer to dry out these spots, and do final sanding. Paint the wall up to the corners. If flat paint is used, you can usually just give a thin coat over the patch before repainting the entire wall. Otherwise, do whatever you do to avoid flashing with sheen paints (priming if necessary).
Regarding prices, you charge what it takes to make money. Of course these are short trips and you'll have to schedule your calendar accordingly. But that's what professional handymen do. If you do it right, you'll have a full or nearly full two days anyway, and this will just be a little extra on those 2 days.