1/2 ply from HD that has the "Sandable" side around 19 bucks a sheet. Used 3/8 rock on the ceilings and painted everything off white with a Sears brand at 7 bucks a gallon.
I was going to use 1/4 ply on the ceiling since it's lighter but 13 bucks compared to 6 it was the rock. I didn't tape or mud. It's a workshop in a detached garage so who cares if it's taped.
Don