I would make a few calls to paint companies and find out how much they charge for the painting of a room (give your measurements), in comparison to the deal they usually offer if you have a few rooms painted at once. Compare the price difference as you get the rates.

Some charge by the hour and other companies charge by the job.

I would recommend hiring a company that charges by the JOB because if they paint S L O W then it can be quite expensive. This way you don't have to concern yourself with how quick they paint lol.

That is how you can price an interior paint job... :D Good luck xo