It can be cheaper to drywall, but it depends on the specific situation. For example, if there is already wiring or plumbing in the ceiling that needs to be hidden and would need to be re-run through the walls if drywall was installed, then it might be cheaper to drop the ceiling. Also, if there is an existing dropped ceiling that is in good condition, it might be cheaper to just install new drywall over top of it instead of taking down the old ceiling. Generally speaking, though, it is often a little bit cheaper to install drywall than to install a drop ceiling.

It depends on what you are comparing it to. If you are comparing it to the cost of labor, then drywall is cheaper. If you are comparing it to the cost of materials, then a drop ceiling is cheaper.