Will require more paint if you're trying to paint a dark room a lighter colour. Note that much of the gear is reusable, so there will be economies if you paint another room. This is an average for an urban centre. Will vary from city to city and depend on experience and reputation of painter. Student painters are often, although not always, less expensive.
Cost assumes painters move furniture in and out of room and do prep work.
Could be a lot longer if you can tackle the job only in spurts. Shopping for paint and associated gear will probably take a couple of hours. Some tradespeople won't care as much as you do about the finished result. If time is more important to you than money, hiring a professional will be smart investment. On the other hand, if saving money is your goal, painting the room yourself cuts your costs neatly in half.