Here's an estimate in the context of the economic output of the area:
Just this partial list — I haven’t included the platform doors, for instance — brings the total to about $111 billion. It’s a big number. But not when you put it in context. New York City and its environs generated $1.7 trillion in gross metropolitan product in 2016. That’s roughly 9 percent of the nation’s overall G.D.P. How much of that activity is dependent on the subway? About a year before Hurricane Sandy, a state-funded group of scientists and engineers produced a comprehensive (and as it happens, prescient) report on the damage that a hundred-year storm surge could cause to the system. One of the study’s authors, Klaus Jacob, a geophysicist at Columbia University, told me that losing the subway for a month would cost the city about $60 billion in lost economic output.
Why is the system starved for cash?
The subway subsists on an ad hoc patchwork of taxes implemented and overseen by a governor who represents millions of voters well beyond the greater metropolitan area. Brennan estimates that New York City is responsible for 55 percent of the state’s revenues, but that doesn’t change a fundamental political reality for its governors: People who don’t ride the subway don’t want to pay for the subway. You might say the subway is a victim of the same rural-urban divide that has come to define American politics today. A more parochial version of this divide exists in the city itself, where representatives of car-centric outer-borough neighborhoods continue to fight congestion-pricing plans that could deliver hundreds of millions of dollars every year to the M.T.A.
That's from an article in the NYTimes Magazine by Jonathan Mahler. On why the subway is important:
The subway’s importance to the city begins with a single, durable economic principle: Cities create density, and density creates growth. Economists call the phenomenon agglomeration. Not only does geographical proximity reduce costs, but it also facilitates the exchange of knowledge and spurs innovation. It’s a principle that holds true for better and worse and regardless of the industry. The free-market economist Edward Glaeser has pointed out that the junk bonds and leveraged buyouts of ’70s and ’80s Wall Street were as much the product of human collaboration as they were of corporate greed. The urban-planning professor Elizabeth Currid-Halkett coined the phrase “the Warhol economy” to describe how this same sort of cross-fertilization and idea-sharing works in New York’s art, fashion and music worlds. As industries grow, they attract and create new, connected ones: Book publishers beget book agents, tech start-ups beget venture-capital firms and so on. It all begins with the ability to pack large numbers of people into small spaces and then unpack them at the end of the day. Without the subway, this process breaks down, and the city dissipates.
Like most good-government tools, zoning sounds boring, but it is in fact a secret means by which cities are shaped and fortunes are made. If the subway delivers density, zoning determines where that density goes by doing things like placing limits on how tall buildings can rise or how many dwellings they can contain. New York’s zoning codes are byzantine, the product of years of pushing and pulling between the desire to allow the city to evolve and grow and the impulse to keep development in check. These codes have helped preserve the city’s historic buildings and neighborhoods while preventing its streets from being forever cast into darkness by endless rows of skyscrapers. But they have also had the effect of restricting the supply of housing, which has driven up prices, especially in neighborhoods with desirable buildings and good subway access. “I rent studio apartments for $3,400 a month,” Walentas told me. “It doesn’t make any sense.”
It's about mobility:
The case for the subway is the case for mobility: physical mobility, economic mobility, social mobility. The business leaders, politicians and engineers who made the subway all those years ago understood that promise, and it remains the most profound message of the system even in its decline: The city can be built, and the people can come, and they can thrive — millions of them, then millions more.
If the subway isn't fixed:
Here’s one possible scenario: New York won’t die, but it will become a different place. It will happen slowly, almost imperceptibly, for years, obscured by the prosperity of the segment of the population that can consistently avoid mass transit. But gradually, an unpleasant and unreliable subway will have a cascading effect on New Yorkers’ relationship with their city. Increasingly, we will retreat; the infinite possibilities of New York will shrink as the distances between neighborhoods seem to grow. In time, businesses will choose to move elsewhere, to cities where public transit is better and housing is cheaper. This will depress real estate values, which will make housing more affordable in the short term. But it will also slow growth and development, which will curtail job prospects and deplete New York’s tax base, limiting its ability to provide for citizens who rely on its public institutions for opportunity. The gap between rich and poor will widen. As the city’s density dissipates, so too will its economic energy. Innovation will happen elsewhere. New York City will be just some city.
For me this is background to a novel I'm reading, New York 2140, by Kim Stanley Robinson. As the title suggests, it's about New York City in the year 2140, after climate change has raised the sea level by 50 feet. I'm 250 pages into a 600 page book. So far, no one travels by subway. Why? Because the city's under water. The tall buildings, of course, rise above, and more of them have been created. People get around by various kinds of water transport and by airbridges between buildings.