cost per square foot is not the issue. possible need to rout plumbing and drainage IS the issue, then how fancy you want to get. Anna is right -bring contractors INTO your home to price what you want done, not over phone.
Sorry - but any anwer you get here is useless and almost certainly misleading, because you said BUILD / ADD a bath, not remodel one. Even a range is still just a wild-goose guess. Get quotes based what needs to be done in YOUR house - then there are fewer suprises.
Coldwell Banker Hearthside Realtor
Having owned properties and added both powder rooms and full baths, speaking as an owner I can assure you that once you do the physical rough out of the space you are talking about a huge variable of options with which to fill it. Do you want to go to Restore and pick up recycled bath parts? Are you heading to the home depot? Or are you installing a high end shower?
Your best bet would be to speak with a few local small contractors that can accurately and professionally assess both the property, your basic needs, and your wish list. I think it is less than accurate to take a ppsf application in this situation. You may get answers, but the reality may vary for you base upon the structural and building issues that arise. Go to the source and get an estimate.
Coldwell Banker Preferred