Whether doing it yourself or hiring someone, buy the best quality paint you can afford; it lasts longer and in some situations requires only one coat of paint instead of two. Painting a room usually involves prep work, primer and latex or alkyd paints with a range of finishes. HGTV.com gives a quick overview [1] as well as a list of painting dos and don'ts[2] .
Basic labor to paint home with favorable site conditions. Prep up to 3 hr per 100 SF - clean, scrape, sand and patch up to 4 defects per 100 SF. Caulk gaps and spot prime. Roll / brush 2 coats of paint over lightly textured ceiling/walls. Paint up to 2 doors and door trim per room. Includes planning, equipment and material acquisition, area preparation and protection, setup and cleanup. 129 square feet $97.99 $132.56 
You might pay an additional $1 to $5 per square foot for optional repairs and updates. Consider having your molding, trim and baseboards checked while you have a professional in your home. This might be a good time to upgrade or repair any cracks, warps or other issues. Pros can usually touch up or replace these items for an affordable rate, and they may even package such fixups with your project.

Take for example, I charge 1.75 sq foot for 10x10 room with 8 foot ceilings. By my calculations this would cost $175 with floor space sq foot, but with you method, it would cost $576.00 That is outrageous. Of course I didn't subtract the windows and doors, which is another thing that makes no sense. Cutting in around those windows takes more time, then just rolling a wall.
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