What should I charge for drywalling a room?

My sister asked me if I would drywall a room in her house for her and she wants me to name a price. I told her I would, but I have no idea how much to charge her for the job. All the materials (sheetrock, joint compound, screws, etc) will be provided by her. She just needs to know what I want for doing it. The room is approximately 14'x20'x9'. I have to tear down the old walls and ceiling, hang sheetrock on the walls and ceiling, and replace a light fixture in the ceiling. I have no idea how long a job like this will take and I don't want to ask too little or too much. So any ideas for what I should charge?