which is cheaper plywood ceiling or pvc ceiling?

Plywood, is my vote. Unless you go with flat or smooth PVC you're looking at no matching replacements in the future and would need to re-do the entire ceiling. And, if you plan to do plywood out of full-size sheets, it can be either finished just like drywall or the other direction as a quality finished hardwood floor...PVC can't do either of those, plastic's a punk.