Paint will run about $40 per gallon depending on quality etc. Depending on how big the room is, you'll likely need 1-2 gallons. I suggest doing it yourself because you'll save on labour, but you should first learn a bit about painting. I started painting as a summer job two years ago and it is harder than it looks. Unless you're not really concerned about it looking good
Oh, and you'll want to prime first.
And check what kind of paint is currently on the walls (acrylic, latex, etc.) cause what you put on top might not stick properly unless you get the same kind.