There is no quick answer to your question. Every professional should tell you that it depends, rather it will be clear span or have load bearing walls.
And, with that said, it will always depend on the overall blueprints and specifications.
Another answer to our question is that in most cases the more you build, the less it is per square foot. If you are building the same bathroom: Tub, 4’x4’ shower, two vanities and a toilet. If they had the same fixtures and finished a 12x20 addition would be more per sq ft than a 20x20 addition.