I exchanged it. I originally paid $289.99+tax for Motorola Droid Razr. I brought back my RAZR 2 months later. They had me return the phone at customer service. I then paid $289.99 for the Galaxy Nexus. I DID NOT pay full retail for the new Nexus. That's my experience. I have no idea what is happening at your Costco.
This makes sense only if you activated a line through them with the ODR and kept that line and only exchanged the phone to the GNex. The reason is they get the same rebate from Verizon for either phone so to them it's a wash.