Goldman: "New York apartment prices are very high relative to the observable fundamentals. Using three alternative yardsticks—price/rent, price/income, and affordability—we find that prices would need to decline by 35%-44% to return to the valuation levels seen in the 1995-1999 period, before the start of the recent boom."
That is, unless things get really bad:
Goldman: "It is instructive to consider the potential implications of a return of relative Manhattan incomes toward the national norm prevailing before the Wall Street boom of the past two decades, either because of pay cuts in the financial industry or because of a possible out-migration of affluent individuals. From 1969 to 1986, Manhattan per-capita income averaged 2 times the national average, with no clear trend. Over the next two decades, however, it grew to 3 times the national average. If incomes fell back to the pre-1986 level of 2 times the national average—and if national per capita income remained unchanged—prices would need to fall as much as 58% to return to the 1995-1999 price/income ratio."