My husband is a painter and the price can vary greatly as it depends on a few things.
Firstly the amount of preparation work involved - what is the colour of the existing walls (dark/light/texture etc), what is the state of the walls, is there alot of scraping, filling of cracks etc. Prep work takes up most of the time, and professional painters will price this in accordingly.
Painting is the quickest part, once again depending on the color of the wall previously and what colour is being painted on. Eg. A house my husband painted had a navy blue ceiling , and they wanted it covered in white. It took, 2 coats of undercoat and then 6 coats of paint to cover.
All this needs to be taken into consideration.
The more prep work you can do yourselves will help save you money. Maybe when you get a quote, ask what prep work you can do yourselves to save money. This also includes moving furniture etc for easy access, as all that takes time.
$5000.00 - $7000.00 is an average price.