Over recent history, NY's high end apartments have been going through the roof. In part because the winners in ascendant economies world-wide would speculate on the top-end apartments there in the tens of millions of dollars as an investment and then often rarely or never use these properties. just holding them for the appreciation.
The problem is the high-end speculation was helping to pull everything else up. Values went up 30% since 2011 with the first sales of properties over $100 million happening in 2015.