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Depnds on what you mean by expensibve and where you want to go. All cruises are going to be more expensive at Christams than they would be if you went on the same cruise a few weeks before or after. I'd suggest going onto the Cruise offers section on this website for December and this will give you an idea of what your options are regarding length of cruise, destination, cabin type and star rating of the cruise line for the budget you have in mind. You may find that the best cabins on the more popular cruises will already be booked up for this Christma.
Cruise and Christmas are an oxymoron. Many cruises getting sold out already, and the more they sell, the more the prices go up...
Maybe not the answer you want be we went on the Liberty of the Seas from Miami pre Christmas last year flying back overnight 22 - 23 Dec and having a quiet Chritmas day at home. Miami and the ship had all the Christmad spirit and decor (and sun instead of the dull dark rain of the UK) but the best bit is aside fromthe overpriced christamd cruises this one was under priced and a fantastic deal (probably because of the timing but it made a wonderful family christmas present that I could afford).
Depending on what accomadation you need, look at Princess,NCL going to Hawaii on the web in Nov - costs this year, was between £100 & £120 pp per night for 16 & 17 night trips. Fred Olsens, Boudicca is very reasonable this year (09) Xmas & N.Y, for an inside cabin, under £100 p.night Including flights. You will have to be quick as they do sell quickly. Great way to spend Christmas though.