Cruising without a butler and a private hot tub is like flying first class without Dom Perignon and a flat-bed seat. Sure, you can snag a cruise to the islands for a song -- try $100 a day or less for a basic cabin on the Vegas-style mega ships -- but the best rooms go for a whole lot more.

The most expensive penthouse suites at sea routinely go north of $1,000 per person per day, and often more than twice that.

The QM2, lavishly christened by the Queen herself in 2004, has a pair of 2,249-square-foot Grand Duplex Suites that include exclusive access to a private sundeck, restaurant and lounge. The price for the royal treatment? Try $3,900 per person per day for a six-night transatlantic crossing.

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Since cruise ships may have hundreds and often thousands of cabins, but only a handful of penthouse suites, it's not surprising the top digs get snapped up first.

"We book from top to bottom, with penthouse accommodations being the first to go," says Bill Smith, Crystal Cruises' senior vice president of sales and marketing.

And don't expect a discount at the top of the food chain.

"The best suites sell and are not normally subject to the pricing carnage seen in lower category accommodations, or at least the extent is less," says Charlie Funk, co-owner of Just Cruisin Plus in Nashville, Tenn.

They sell fast, because they're luxurious and because of where they go. The most expensive penthouse fares tend to be for the more exotic locations. On the Crystal Serenity and Seabourn Pride, it's an 18-night cruise around Cape Horn to Antarctica, while the top on the Silver Shadow is a week sailing between Stockholm and Copenhagen.

"Penthouse prices are going up in the Mediterranean and Alaska," says Michael Driscoll, editor of industry bible Cruise Week, adding "but like everything else in the Caribbean, penthouse rates are flat or down in that market due to overcapacity."

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Who can't overlook a been-there-done-that Caribbean itinerary when there's 5,750 square feet of living space to explore? That's the size of the pricey three-bedroom Garden Villas on the new Norwegian Pearl, which run $2,000-plus per person per day.

For some, size is all that matters.

"Our clients want the space, and in most cases also want the two bathrooms that the largest suites can provide," says Mary Jean Tully, Chairman and CEO of Toronto-based Cruise Professionals.

Of course, "big" is a relative term. The owner's suite aboard the SeaDream I is just 450 square feet, but on a yacht-like vessel that only carries 110 passengers, this is exceedingly generous.

Size aside, luxury is all about the right amenities. Walk-in closets, stocked mini bars, Internet access and flat-screen televisions with CD/DVD players are penthouse basics. The top cabins on Mariner of the Seas, for instance, have an iPod player with Bose or Logitech speakers. Like the best hotels, fuzzy robes, pillow menus and 24-hour room service, including ordering off the restaurant menus during meal times, are the norm at the high end.

So are the services of a butler and concierge who have chilled champagne ready when you arrive, deliver canap�s before dinner, unpack suitcases and make spa or shore excursion reservations.

For some, the hook is a bit more esoteric. To the tune $1,295 per person per day, the amenities in the huge Penthouse Suite on Celebrity's 1,950-passenger Millennium include a baby grand piano.

"Our suites are on par with the best accommodations and amenities of the luxury lines, but Celebrity ships also have the additional space to offer guests that much more variety in venues, dining options, entertainment options and more," says Dan Hanrahan, president of Celebrity Cruises.

Big ship or small, cruise fares always include three-plus meals a day, along with entertainment and a variety of activities. Smaller, more luxurious ships are typically more inclusive, with open bar and gratuities also built into the price.

"If you compare our cruises fares to the cost of a comparable luxury hotel and resort vacation, our prices are still very competitive and in many cases probably the best value for the money spent," says Mark Conroy, president of Regent.

To come up with our list of most expensive cruise ship penthouses, we looked at worldwide itineraries without regard to high or low season, which varies depending in different parts of the world (summer in Europe, winter in the Caribbean and so forth).

For the penthouses listed, count on even the least expensive itineraries still running at least $1,000 per person per night. We did not include condo cruise ships like Residensea or yachts geared only to full charters, since they are not cruise ships in the traditional sense. All rates are per person per day and based on double occupancy.

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