The swankiest, most expensive condominium building in Manhattan is called One57. Its six-bedroom, 11,000-square-foot duplex penthouse, 1,000 feet above 57th Street overlooking Central Park, recently sold for an unprecedented $100.5 million.

And best of all, the owner gets a 95 percent break on property taxes — thanks to a New York City program that is supposed to generate affordable housing.

It’s called the 421-a program, and it gives developers of new apartment buildings up to twenty years of free or greatly reduced real estate taxes. In most of the city, developers get the tax break without building a single affordable apartment. They don’t even have to apply or submit any documentation to be approved.

One building got $23 million in tax breaks from the 421-a program for 306 luxury apartments and 8 affordable ones. In other words, each affordable apartment cost $642,000 in public subsidies. As Maritza Silva-Farrell of the Real Affordability for All Campaign told a Gothamist reporter, “With that amount of money, the city could buy each of the people in those units a condo.”

In the very hottest Manhattan and Brooklyn neighborhoods, 20 percent of the 421-a subsidized apartments have to be “affordable.” The other 80 percent can be market rate, which usually means luxury housing.

But even the “affordable” units are reserved for a family of four earning up to $51,540 a year in a city where one-third of households make $33,560 a year or less. According to the Association for Neighborhood and Housing Development, 153,121 apartments got a 421-a tax break in the last fiscal year, but only 12,748, or 8.6 percent, of those were affordable.

This amounted to $1.1 billion in lost revenue that could have been spent on schools, public transportation, or hospitals. Or affordable housing, for that matter — another $1.1 billion could have paid for 100,000 new rent subsidy vouchers to provide many of the city’s poorest with housing.

The good news is that the 421-a program will expire on June 15 unless it’s renewed by the New York State Legislature. The bad news is that New York City’s liberal Democratic Mayor Bill de Blasio doesn’t want it repealed.

Tenant advocates have been working hard to end this massive subsidy for the very rich. But de Blasio only wants to tweak it slightly.

The mayor says all developers who get the tax break should have to create 20 percent of “affordable” housing in their projects. In the most fashionable and profitable areas of Manhattan and Brooklyn, they’d have to build 30 percent affordable housing. In return, he’s promising landlords an extended tax break from the city — because apparently twenty years just isn’t long enough.

In a New York Daily News article, David R. Jones, president of the Community Service Society, writes:

It would be much better to rip up 421-a and start over. . . . Even if these ideas make it through the legislative sausage-making process, they will increase the program’s efficiency only modestly. They won’t change the fundamental mismatch between a broad, no-questions-asked exemption and the goal of producing much more housing New Yorkers can afford.

De Blasio’s problem is that his entire affordable housing program, which did so much to get him elected, depends on the goodwill of the real estate barons. He’s promised to get them to build 80,000 affordable apartments and preserve 120,000 more. But unless these wealthy developers can make enormous profits, they won’t build anything, and de Blasio’s whole plan goes down the tubes. As Capital New York observed, “His agenda is, in many ways, in developers’ hands.”

And the developers want to hang on to their 421-a tax breaks. “Without this critical tax incentive,” Steven Spinola, president of the Real Estate Board of New York, told Capital New York, “the city would see a sharp drop off in the production of new housing units.”

De Blasio had a stark choice on the 421-a program. He could either side with the developers and landlords or with the tenants who need affordable housing. The mayor has come down firmly on the side of the real estate millionaires.

De Blasio might like to build all the housing he knows New Yorkers desperately need, but his entire affordable housing plan is based on bribing developers. Any time they devote even 20 percent of the apartments in a new building to affordable housing, they get to charge market rates that few, if any, working people can afford for the other 80 percent. The 421-a program is just another way for de Blasio to pay off developers to build a few affordable apartments.

And even in the very unlikely event that de Blasio succeeds in developing or preserving 200,000 affordable units, he won’t come close to meeting New York City’s need for affordable housing.

For example, nearly 61,000 homeless people, including 14,524 families with 25,459 children, are sleeping in New York City shelters every night. There’s no way they are ever going to be able to make the rent, even in the affordable housing units that de Blasio is promising.

More than 2 million people in New York City are spending at least half of their monthly income on rent. Almost 5 million pay more than 30 percent — the level at which rent is considered affordable. There are more than two desperately poor families in the city for every apartment they could afford.

The only long-range answer to New York City’s housing crisis is pressing the federal government to build new public housing. But de Blasio has already ruled that out because there’s currently no funding available.

This is typical of de Blasio. He proposes all sorts of great things like affordable housing, decent public schools, and a $15-an-hour minimum wage. He just won’t fight for them. Instead, he keeps on playing by the rules of the establishment political game. When this means he has to accept the 421-a program with its billion-dollar subsidies for the richest real estate barons, it’s a price he’s willing to pay.

If poor and working-class New Yorkers want anything more, we’ll just have to fight for it ourselves.