I just went through the motions of booking the 6/15 Med cruise on Costco Travel's site. The price for a 9C room is $10,044. The cash card for that cruise would be $880, which is just over 8%. Here is the fine print regarding the cash card:
The exact amount of the benefit will be calculated during the online booking process. The Costco Cash Card offer is one per stateroom based on double occupancy, is valid for dates specified and is only valid for bookings made through Costco Travel. Offer is nontransferable and may not be combined with any other offer or coupon. The card, which you'll receive approximately three weeks after you return from your cruise, may be used toward merchandise or gas at any Costco warehouse throughout the United States, or toward online purchases at costco.com. The only way lost cards can be replaced is by presenting your original vacation or cruise invoice to Costco Travel. Neither CWC-Gift Card Co. nor any of its affiliates are responsible for use of this card without your permission. Gift card balances cannot be applied as payments to any account or redeemed for cash, unless required by law. Costco Cash Cards cannot be used for travel purchases.