Homeowners usually ask for a price per square foot, but that's not how a painting estimate is really built. The cost of painting a room comes from four things: how much prep the walls need, how many surfaces are included, how many colors you're using, and how much furniture and trim has to be worked around.
Prep is the biggest variable. A room with a few nail holes takes an hour to patch and sand. A room with water stains, cracked corners, and old wallpaper glue takes a day. That's why an honest estimate starts with someone looking at your walls, not with a formula.
Surfaces multiply the work. Walls only is the cheapest job. Add ceilings and the labor roughly grows by a third. Add trim, doors, and closets, and it grows again, because trim is slow, detailed brush work. Each color also adds cut-in time, so one wall color and one trim color is the most efficient combination.
When you get an estimate, look for the products named, the number of coats, and which surfaces are included. Two coats of a mid-grade washable paint over properly prepped walls will look better and last longer than one thick coat of premium paint over dust. If an estimate doesn't say how many coats, ask.