From the NYT:

The penthouse at One57, which offers panoramic views from 1,000 feet above 57th Street, recently sold for a record-setting $100.5 million.

But it is not the price that has grabbed the attention of housing advocates, policy analysts, developers and city officials. Rather, it is one of peculiarities of New York real estate: a billionaire’s lair that comes with an incentive that cuts this year’s property tax bill by 95 percent, or an estimated $360,000.

That has turned the six-bedroom, 11,000-square-foot duplex into a prime example for an intensifying debate over the future of a housing program known as 421-a. It offers generous property tax abatements for as long as 25 years to encourage construction, or in some cases, to generate apartments affordable to poor and moderate-income tenants. …

According to city records, about 150,000 apartments got the 421-a tax exemptions in the fiscal year 2013, at a cost of $1.06 billion in forgiven taxes. The tax abatement, which starts with a steep, 95 percent discount on property taxes, slowly decreases over time until the tax hits full rate.

But only 12,748 of the 150,000 apartments were earmarked for low- and moderate-income tenants, making it a costly way of creating more affordable housing, according to the Association for Neighborhood and Housing Development, an advocacy group.