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  as well as a list of painting dos and don'ts .
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.
YP - The Real Yellow PagesSM - helps you find the right local businesses to meet your specific needs. Search results are sorted by a combination of factors to give you a set of choices in response to your search criteria. These factors are similar to those you might use to determine which business to select from a local Yellow Pages directory, including proximity to where you are searching, expertise in the specific services or products you need, and comprehensive business information to help evaluate a business's suitability for you. “Preferred” listings, or those with featured website buttons, indicate YP advertisers who directly provide information about their businesses to help consumers make more informed buying decisions. YP advertisers receive higher placement in the default ordering of search results and may appear in sponsored listings on the top, side, or bottom of the search results page.