I'm not sure I understand your question. Drywall can vary depending on how much needs to be replaced. Also drywall is more expensive when redoing areas with moisture, such as a bathroom as you are using green board or cement board. Also you have to add the cost of tape and spackle. You have to add the cost of labor too if you aren't doing it yourself.
My advice would be to get an estimate from a contractor.
As for your other questions, without an address, I cannot answer them.