The exterior of One57. Source: One57.
Bill Ackman, the billionaire hedge funder, told The New York Times that he and a group of friends bought one of the penthouses "just for fun"—and for flipping.
But the reality of One57's sales may be a little less stratospheric than its image.
Read MoreSecret mansion buyers hide behind LLCs
According to public filings from One57's developer, the building only sold two units in the entire first half of 2014. Yes, about 75 percent of the building's 92 units are sold, but at the current sales rate, it would take more than six years to sell the remaining units, according to Jonathan Miller, president and CEO of appraisal company Miller Samuel.
And that's at a time when a half dozen other big condo megaprojects are going up nearby, including 432 Park which boasts higher floors.
Prices for new luxury developments in Manhattan fell 8 percent in the third quarter, according to a report from real estate management firm Douglas Elliman. At the same time, listing inventory from new development has nearly doubled compared with the year-ago quarter, to about 1,400 units.
Read MoreLife's a beach: Super rich invest in island life
The developer of One57, Extell Development, doesn't disclose sales figures.
"Almost all of our projects slow down at the 70 percent to 80 percent mark, until we can complete the building and show the units finished and clean," said Gary Barnett, Extell's president.
He did, however, acknowledge the possible effects of growing inventory and competition, saying that "there are certainly more buildings that are being built now that are slowing potential sales."