It really depends on the room. Are there also moldings or chair rails that will have to be taped and painted around? Do you need them painted too- white or something like that? Is he painting the ceiling? Are you guys moving all the furniture out or is he supposed to cover it and paint around it? How much patching of holes or cracks needs done?
Around here $300 sounds about average if there is also trim to be taped and then painted and if it is a medium sized room with a few holes to patch up. I am sure there are people doing it a little cheaper, but not by very much. The other thing to consider is - are you providing the equipment - brushes, rollers, trays, tape, etc. or just the paint and he is bringing all the other stuff with him?
It always SEEMS cheaper to say " You should just do that yourselves!" but it can really depend on if you already have all the STUFF to do the job, or can borrow it from someone for free.
Once you buy all that, the paint AND factor in how long it will take you to tape up all the trim and do the edging and put on two coats... if you haven't painted much before, it can take MUCH more time than you think and you might have gotten a better looking, more professional job for what it costs you in the end to do all the work yourself.
Good luck either way!! :)