That really depends on you. There are so many painting techniques, not to mention the various brands of paint. You may get a gallon of paint for $9.00 but you could just as easily spend $30.00 on a gallon. Naturally, if you contract a painter, it will be more expensive. But at least you won't have to clean up any mess!
The cost to paint a room will carry greatly based on the how much preparation of the surface is needed, the size of the room and the quality of the paint used. It also depends if you are asking about the price to do it yourself or the total costs of hiring somebody else. A 12 foot X 12 foot room will require 2 gallons of paint. Good paint costs $30- $50 a gallon. You will need some more supplies. For this room plan on $100-$200 to do it your self or twice that to hire somebody else to do it. You can find more information here: http://homeimprovement.lovetoknow.com/Cost_to_Paint_Interior_of_a_House