The value of the gift card you get actually depends on what you pay for your cruise. Of course, the price of the cruise does reflect in the length of cruise and type of cabin. I have had several price reductions on our RCL cruises and have had the GC value reduced a few times. Rather save a few hundred on the cruise and loss $10 on the GC but they always want me to be aware of the change. The price paid is the main reason I'm getting $500 for two weeks in a balcony cabin on RCL and $1600 for three ocean view cabins for one week on
DCL.
I have found that while the cruises offered through Costco travel are a good value because they are the same cost as the cruise line directly, the other travel options are seldom very good. I have always found a better price somewhere else even when I include the GC so keep that in mind if your looking for a AI through Costco.