Not a single penny actually.
Of course that's because decorators don't actually do any work. They walk around and come up with ideas and tell others what they want done(and no, I'm not kidding).
Now I have no idea where you are and painters charges vary a LOT depending on where you are. I live at the beach in N.C. and if the ceiling is flat and there isn't too much furniture to move(we cover a single couch or chair and paint over top of it from both sides. It's large groups of furniture or really big pieces that have to be moved).
Then I usually tell the client what type and how much paint to buy and I charge .50 cent per square foot(beach is a little more expensive than going inland say 50 miles where it's cheaper to live and only costs .35-.40 a square foot). Of course if the ceiling is textured and has never been painted before(like popcorn texture) the cost is $1.25 a square foot Because the first coat has to be applied very slowly and we have to wait while it dries and then paint it again.
In areas like boston and new york I believe the price is around 3-6 times higher due to cost of living and unions.
Oh and NEVER let a painter charge you by how long he spends painting. There is no better way to get screwed than paying a painter by the hour. He has no incentive to get off his butt and actually work hard. He will take his time and "milk it" as we call it to make the most money from you.
First: Is it just for one room or the whole house?..
The contractor will base his pay on the amount of time he or she spends on the room, plus the cost of the paint. if the room is approx 12 x 12 , on average it would cost 100 - 300 depending on time , labor and equipment needed to complete the job.
Best thing to do, Is if you can do the job yourself, or have a "painting party" and invite friends over. (refreshments, beer, food etc) At least you'll save money, and try to have a good time at it.
Firstly there is no such thing as an hour job like number 2 suiggest ;you will need to cover and or move furniture and there is also no such thing as one coat-ide get 300 buxx for any size ceiling not to exceed 200 square ft ;if larger si is the paycheck and you supply the paint.
I would charge a days labour, but there are alsorts of people who would charge you much less. So, in the UK that means you would get quotes between £50 and £150 plus paint. Although I stopped a few years ago, so it might be a little more now.