For the princess cruise we are taking, the 3rd and 4th passenger cruise rate was less than half of the 1st/2nd passenger rate, before taxes. All 4 then had the same 'non-commissionable fare' charges. Last year we looked into a particular cruise but all of the triple/quad occ rooms were booked already.
Thanks for all the great answers from all posts. This just demonstrates how helpful this website has become. My family has decided to book 2 cabins since my mom is celebrating her retirement.