Because I have an executive membership so I get some cash back.

I checked vacationstogo, directly with the cruise line and Costco, and all 3 were within a few dollars of each other.

You probably won't know the "true cost" til you go through the whole booking process. The list price on our 7 day carribean cruise in March was like $600/person for 2 people. Once we paid for it, it was like $900 a person after all the taxes and fees, etc