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.