Are you a member at Costco?
On the website they had a special for the Riviera (not including airfare) thru Dec 20 for 5 nights at $632 a person.
http://www.costco.com/Browse/Product...ath=1133*2552*
Airfare is bookable for an additional fee - but it looks like it might be a good place to start if you call the reservations number
Call 1.877.849.2730
and see if they have anything in your time window. The membership fee is like $40 here, so even if you are not a member, that extra bit still might save you a good deal. My parents arranged a cruise through Costco and said it was a great deal.