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.

At a City Council hearing last week, critics derided the 421-a program as an expensive boondoggle, a giveaway to developers building luxury housing in a city where the poor and the middle class often find themselves priced out of the market.