Among all the items billionaires buy with their money, high-class mega-yachts dominate the top-10 list of the most expensive luxury purchases, according to Wealth-X.
Luxury vessels take up eight of the top 10 slots, with the world's largest mega yacht, the Azzam, ranking at the top of the list.
The 590-foot yacht was purchased in 2013 for a reported $627 million by the ruler of Abu Dhabi, Sheikh Khalifa bin Zayed al Nahyan – a price that would mean the vessel is worth around 3.5 percent of the Sheikh's his net worth of around $17.9 billion, said Wealth-X, which researches the ultra-wealthy.
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The $627 million price-tag for the Azzam, which is also the world's fastest yacht over 300 feet long with a top speed of 37 miles per hour, overtook the previous largest yacht, the $485 million Eclipse, Wealth-X said. Owned by Russian tycoon Roman Abramovich, the Eclipse was ranked number three on the list of most expensive luxury assets, coming in at around 4 percent of the total estimated net worth of Abramovich.
The Eclipse features two helicopter pads, 11 guest cabins, two swimming pools, exterior fireplace and a dance hall, as well as coming equipped with intruder detection systems, a German-built missile defense system, and bullet-proof glass and armor plating in the master suite and bridge. It can accommodate up to 30 guests and 75 crew.
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The 440-foot Serene, which features seven decks, two helicopter landing platforms, storage for a large submarine and a large internal swimming pool, ranks at number four on Wealth-X's list. The yacht, owned by Yury Shefler is valued at $330 million, suggesting the founder of the company that makes Stolichnaya Vodka stretched his finances a bit for the purchase as it would mark around 20 percent of his estimated net worth, according to the Wealth-X list.
To be sure, real estate can't be written off. The second-most-expensive luxury asset is Lanai Island in Hawaii, purchased by Oracle founder Larry Ellison for $500 million and representing around 1 percent of his estimated net worth of $46.4 billion.
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Lest you worry that Ellison isn't cruising the high seas with his fellow billionaires, he is part owner of the 454-foot Rising Sun. Although it doesn't rank on Wealth-X's top-10 list, the yacht houses 16 guests and 45 crew and cost an estimated $200 million to build.
The Monaco home of Dmitry Rybolovlev tied with another yacht for eighth place on the list, with the real estate having a price tag of $286 million, coming in at 2.5 percent of his estimated net worth.
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But while yachts may rank among the most expensive toys, their sales and prices have been on a downtrend.
Deliveries of superyachts—those more than 30 meters, or around 98 feet—fell to 169 in 2012. This was down from the boom-time level of 261 seen in 2008, according to the Yachting Index from Superyacht Times and Camper & Nicholsons.
Overall, prices are down about 20 percent since 2008, and the number of superyachts being built has also fallen.
Some Russian and Middle Eastern buyers of very large yachts—those over 250 feet—have continued to place orders. The average size of yachts ordered also continues to increase. But the mass of U.S. boating enthusiasts who were buying 120-foot and 150-foot boats in the peak years of 2007 and 2008 has yet to return.
—By CNBC.Com's Leslie Shaffer; Follow her on Twitter @LeslieShaffer1