As a rule of thumb I charge at least 1&1/2x the outside as I've never had an inside to do yet that has not taken longer than the outsides. But I do take each job on it's own. Eg:. I had one job where I did the whole of the outside + the inside of the living room, front and back and the inside of the front porch and inner door. Because one person in the household was a heavy smoker the inside part of the job took as long as the whole of the outsides. They were priced accordingly