As far as I know Disney doesn't allow Costco or any other TA to discount their cruises. Are you sure about this???
Yes I am sure that you pay less with Costco. You pay the same for the cruise but they give you a cash card that is 9% of the cost of the cruise. The net amount means you pay less with Costco. This is complete nonsense. Why should you pay less with Costco than if you book direct?