What to charge painting per square foot?

Short answer? Enough to cover all the materials and your time at a decent wage.
So the long answer all depends on your overhead of rent, insurance, transportation, and also depends on your skill level. A pro can actually get away with charging less because of the true benefit of experience to go fast while hitting every detail.
Warning...If anything in the work to be completed will be new to you or pose a problem then ADD more to the charges. Do NOT discount if your doing something new.. it will only get you in trouble.
On flat wall painting, smooth surface, previously painted with a similar coating, not a lot of 'cutting' and trim to mask or avoid....(something that no one would ever need you to do for them) you could charge as low as $0.50/sq.ft. That is wall square footage and is equivalent to getting $160.00 + tape and paint costs for the service of painting a 10 x10 office with no windows or door trim and 8 ft. ceiling in about 3 hours. Don't forget to charge a little if your consulting for color, picking up the paint, or moving furniture. Every task adds some cents per square foot. ( I prefer to charge by the 1/2 day) People absolutely hate painting, and will pay to avoid it. They will pay better if you do a meticulous job.
Most important in price is the level of perfection they expect at the end of the job. Do not let a person fool you into thinking that they just want a quick job where you could charge a minimum price. You may you look bad to put your name on the quicky / detail missing job even if the customer were happy. If you do not pin down what is really desired, you will be on the hook for all the little extra's that they didn't know to ask for. "Your the pro, you should have known, and you set the price." is how it might come out in court.
Most often the customer wants to see THE FINISHED PRODUCT, with holes patched, previous painters errors corrected, priming when recommended, quality full coverage paints, two coats for extra durability and all. You can cut unneeded expenses like primer or patching if the existing wall is sound and smooth. Or 1 coat it if your repainting the same color.
Painting a similar 10x10 room with 2 windows of fancy wood trim, crown wood trim against the ceiling, newly installed carpeting and a person who wants the perfect color to be just right an look new "like it should" and they complain about the last painter or their husband (as most of my customers are married women). This person gets what they want.... Patching, sanding, priming, caulking against the trim to hide gaps, taping where needed to avoid paint splatter, Primer and 2 coats of paint, very close trim cutting and a vacuming to remove the dust from sanding and an extra charge in case anything goes wrong. 2008 price - 800 for about 2 long days of labor + the cost of the materials. Make some sq./ft. numbers out of that scenario.
Not that the person is bad or gets charged for being a pain in the personality... but if they are giving extra scrutiny or even making problems, even by asking for something impossible, then you will be spending extra time to make them feel right. You're done when they're done with you, YOU ARE NOT DONE WHEN YOU PICK UP AND LEAVE. You are in the business of charging enough to pay yourself to make them happy. Them being Homeowner, Contractor, Corporation, Business owener, whatever.