Jaw-dropping penthouses and mega mansions are back.

In 2014, there were more sales of U.S. homes worth between $50 million and $99 million than in the previous 10 years combined. Twenty homes in that price range sold in 2014, compared with just one sale (and two more under contract) in 2013, according to sales data from public records tracked by Miller Samuel Real Estate Appraisers. There were four such sales in 2012 (with one more still under contract), two in 2011 and one in 2010, the firm says.

And there were three homes sold in the U.S. last year worth $100 million or more versus just one in each of the previous three years, according to sales data from public records tracked by Miller Samuel. (Some homes were bought before the building was completed, so are still technically under contract.)

Much of this activity, of course, is taking place in and around New York City’s soaring residential property market.

The Mega-Mansion Is Back

In the $10 million to $49 million range, there were an average of 60 property sales per quarter in 2014 in New York alone — triple the 20 transactions per quarter for homes in that price range in New York in the previous decade, says Jonathan Miller, president and CEO of Miller Samuel. “We’ve had this steady market share of $10 million transactions for a long time,” he adds. “There’s a tremendous amount of product in transaction that will close in 2015. I’m not suggesting it will continue rising, but 60 sales per quarter is a new plateau.” Last year, an ocean-view East Hampton property with 16 acres sold for $147 million, http://easthamptonstar.com/Real-Estate/147-Million-Record-Sale-Further-Lane the largest sum ever paid for a single home in the U.S.

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The economic problems in Europe and Russia, the falling value of the ruble and euro in relation to the dollar, and concerns about the Chinese housing bubble have caused many high net worth individuals to diversify their wealth by buying more real estate in the U.S. “It’s a fairly even divide of U.S. and foreign buyers,” says Miller. “We now have this tall tower phenomenon.” A new market has essentially been created with these residential skyscrapers like 520 Park Avenue in New York City, he adds. “I refer to them as the most expensive safe-deposit boxes in the world.” (New York is rated No. 6 of the world’s best-performing residential property markets, behind Monaco, Hong Kong, London, Singapore and Geneva, according to the 2014 “Wealth Report” (pdf) by global property consultancy Knight Frank.)

But buying a residence that costs over $50 million may not be the best solution for the superrich, says Leonard Baron, a real estate analyst and blogger at ProfessorBaron.com. “The reality is if you buy a $100 million personal residence, you’re sinking that money into an asset that brings you no income and costs you money to have it,” he says. “You could have sunk that into a real estate investment trust that could pay you millions of dollars a year.” To get a fair rate of return on your money and cover a tax bill that could reach $2 million or $3 million a year, excluding upkeep, real estate costing tens of millions of dollars needs to increase in value by 12% a year to be a worthwhile investment, he says. “It’s great that they’re buying all this real estate, but they could have used this money elsewhere. Buying homes of this size just takes money out of their bank account.”