Don't hesitate to ask all the questions you need to feel comfortable hiring a professional. You can also ask 3-4 professionals to make sure you have the right quote in mind. You must make sure they have a warranty or some kind of paperwork stipulating the quality of their jobs. Insurance is also a #1 priority to consider, as damage can happen in the process of painting.
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.
I can't say enough great things about Saints of Paint. They went above and beyond my expectations in every possible way. They painted the entire interior of my house and the work that they did was perfect, thorough, and very reasonably priced. Not one detail was missed. On top of that, each member of the team is very friendly, honest, and considerate.
"I had Sam and his team come in to repaint our kitchen cabinets that were originally a dark wood stain. He cleaned, sanded, primed, and put on 2 coats of sheer white on both the doors and frames of the cabinet and they came out great. The kitchen is much brighter and even up close it is hard to tell that the original cabinets were not white. They were prompt and very communicative over the phone. We may soon hire them back to paint the kitchen next."