Mexican billionaire Carlos Slim put his Manhattan townhouse on the market for $80 million, almost double what he paid for the Fifth Avenue property in 2010.

The 20,000-square-foot (1,900-square-meter) Duke Semans Mansion, at 1009 Fifth Ave., was listed Tuesday by Sotheby’s International Realty. A deal at the asking price would be the most expensive ever for a townhouse in New York City, topping a $53 million purchase completed in 2006, said Jonathan Miller, president of New York appraiser Miller Samuel Inc.

Wealthy buyers seeking investments are paying ever-higher prices for Manhattan trophy homes. Billionaire investor Leonard Blavatnik bought an apartment in March at 834 Fifth Ave. for $77.5 million, a record for a co-op. In the same month, a group led by hedge-fund manager Bill Ackman completed the purchase of a duplex at Extell Development Co.’s One57 for $91.5 million. In the city’s history, only one deal was bigger: the $100.5 million sale of a penthouse in the same building.

“When you see numbers like $100 million, $91.5 million and a couple of $70 millions, you start thinking, ‘Perhaps this is in that league,’” Miller, a Bloomberg View contributor, said of Slim’s listing.

Slim, the world’s third-richest person, bought the Beaux Arts-style home for $44 million in July 2010, as Manhattan prices started climbing after the property rout. He said at the time he considered the house an investment and wouldn’t live there.

“This house was bought through a real estate firm in our group that’s dedicated to this type of investment,” Arturo Elias Ayub, a spokesman for Slim, said in an e-mail. “We think it was a good investment for the company to buy it at that time, just as we think this is a good time to sell it.”

Serena Boardman, the Sotheby’s broker listing the property, didn’t immediately respond to a phone message left at her office. The listing was reported late Tuesday by the New York Daily News.

The red brick and limestone mansion near 82nd Street was completed in 1901. Interiors of the eight-story property have hand-carved wood paneling, gold leaf-trimmed fixtures and intricate plaster friezes, according to the listing.