one further question – what color are the walls right now?
If they are dark, then the painter will put a coat of primer on them, then have to wait for that to dry, and then put the final coat on.
If the walls are light, and all the painter is going to do is putt a single coat on, then it’s half the work.
Assuming a 15×15 room (225 total SF) one door, regular height walls, not painting the ceiling, for a regular painter, around $180–220 plus the cost of the paint. More if they have to do a second coat.