It is hit or miss about this. I booked through Costco last year through my parent's membership. When I called back to pay off the balance they would ask for my membership number and would not go any furtner without it. Other times it wasn't a problem. I think I booked 6 cabins under my Mom's name.
I'm a member of Costco, Sams, Bj's and AAA and still found Dreams Unlimited to be the cheapest for an Eastern trip, category 8 outside porthole stateroom about 2 weeks ago. They were also cheaper for a Varandah quote. Check them out before you book. Costco was next cheapest, but was almost $400 more and that was without insurance (DU quote included insurance).
There are so many factors that determine what price a TA will quote.....it is best to check several TA's for the time you want to cruise because at different times different TA's will have a better price for one reason or another....I don't think anyone can claim to ALWAYS have the lowest price....because so many things factor in, not everyone's savings will be the same.
That being said I booked with DCL for the repo cruise the first day they opened up ressies and then shopped around other TA's for price and found Costco to be the best at that time...saved about $500.00 over DCL.