Last spring on the Paul Gaugin in Polynesia there was a woman who was on her first cruise. Thus, she didn't have any of the, uh, biases about various cruise lines that many of us have.
Before choosing which cruise to take, she did a spreadsheet of the PG versus Princess and came up with a difference of a couple of hundred dollars when all was accounted for.
Having spent a week on the PG with her and her husband, I would guess that since neither seemed to be heavy partiers and/or drinkers the difference in cost probably came in less than this. Maybe zero.
-- David