For minimal prep work and one entry door, one closet door and one window, I charge .33 per sq ft per coat. For example, a 12x12 room with 8 ft ceilings is 384 square ft. So if I am painting one coat on the walls (which I never do unless it's the same color), then I would charge $126.72. That includes the materials unless it's a $50/gal paint, then I bump it up $20. If I am doing the trim, then I would charge .66 per square ft for one coat on everything. So one coat on trim and walls would be $253.44. 2 coats on everything would be $1.32/sq ft. This is where there is none to minimal prep work. This is a short cut way of figuring rooms but you have to pay attention to variables such as louvered doors, or French doors which are more time consuming and how much sanding, caulking, etc that you have to do. These prices are not unreasonable especially when you factor in all the costs of running your business such as workman's comp, liability insurance, tool repair and replacement, travel expenses, expenses and upkeep on vehicles, etc, etc. And you still have to eat and pay rent or mortgage. Depending on your overhead, these figures can be low.