Hey steve dave be weary of "builder's grade" paint in some areas. Use good crap in bathrooms (Kilz) and other high traffic areas.
Also, just rough ridin' tile from the top of your showers/baths to the ceiling. Don't put paint there.
Honestly, this should be code. Stupid to do just paint, regardless of the grade.
As far as cheap vs expensive paint, the biggest issues is actually thickness of coat. Many painters do as thin of a coat as possible and usually only one. They tint their primer so that if it shows through it isn't very obvious. It isn't an issue if you are like I am in that the first thing you do in a house is completely change the color basically every where and then do it all again every few years.
Also, don't get flat paint. they will say it is easier to clean and touch up, but my personal experience says that is rough ridin' stupid. Get a satin finish. Not horribly glossy, cleans easy, and I have never had much issue touching up.