Consider purchasing supplies personally to save money. Ask the painter for a bid that separates labor and materials. Then explain that you'll purchase the materials and ask for a list of exactly what will be needed to complete the job. Caulking, for example, is an extra supply commonly used to fill any cracks or damaged areas in your walls -- and one that might be overlooked in an incomplete list.
The cost to paint an average size room (10X12) ranges from $380-$790, not including ceilings, trim or cost of the paint. DIY, this can cost between $200-$300. When estimating the cost, painters will start with how much paint will be required to complete the job. To know this, they first need to know the size of the area to be covered. The easiest way to calculate the size of the paintable area is to add together the length of the wall and multiply it by the width of the room from floor to ceiling. The resulting amount is the room's square footage. This total is a starting point, as it is not all paintable surface. The painters won't paint the windows and doors, for example, and they must account for ceiling trim and baseboards as well. The next step is to subtract the area of the doors and windows -- along with the square footage of the trim and baseboards -- to get an average. Then, use the same calculation (length x width) on the windows, doors, trim and baseboards, and subtract all of those numbers. The result of this equation is the square footage of the room's walls. This is your surface area number. (Painters will typically add a little square footage back in to this amount to account for extra paint, which may be required.)
If you're going to do any part of the painting or prep work on your own, you must know that the first step in preparing a surface is washing it. Since dirt can affect the smoothness of a surface area -- and therefore paint's adhesion to it -- use soap to remove any dirt or stains. Make sure there's no soap left on the walls when you're finished; also make sure to remove any gloss with sandpaper and vacuum up the leftover dust.
I am a painting contractor and have been since 2001. Make sure the estimate provides in writing: What is EXCLUDED as well as INCLUDED. It should state the manufacturer and type of paint going to be used. Estimate says ALL LABOR AND MATERIALS. My estimates to my customers say "guaranteed coverage" eliminates the conversations of 1 coat vs 2 coat. I have my customers submit colors 5 days prior to start date. Customers need to inform me if they are going to use pure white, dark reds, oranges, and bright yellows they need to inform me in that 5 day window, so I can adjust my pricing for 3 coats. Although this more uncommon now than years past because a lot of paint manufacturers have primer with paint products. Let the contractor know if your doing accent walls. This takes longer to cut in straight lines and it requires the contractor to purchase more paint. If you add anything on the scope of work have the painter write out the description and cost prior to them doing the work. Have the estimate say how many days it will take to perform the work. Ask how many workers will be doing the job. Make sure to enforce that number of workers their everyday until the job is complete. Do not give final payment until you do a final walk through. Walk the job when its almost complete and point out areas that you want fixed prior to the contractors final walk through. Its best to do while the workers are still in that particular area as they will have tarps down and areas covered and it will be easier for them to take care of. Purchase a roll of blue tape and stick it to areas that you want fixed. This is called a punch list.