The city plans to spend $374 million to expand a green space near Hudson Yards by three blocks. At more than $124 million per acre, that would make the extension the costliest park project in local history.

Last month the mayor gave a city-controlled corporation the green light to float $500 million worth of bonds backed by property tax revenue from the surrounding area. The Hudson Yards Infrastructure Corp. would use the majority of that to pay for the second phase of a park running parallel and between 10th and 11th avenues as envisioned in the 2005 Hudson Yards rezoning. The first three blocks of Hudson Boulevard Park, between West 33rd and West 36th streets, opened in 2015. The $374 million would extend the park to West 39th Street.

"That is an astounding price tag," said Adrian Benepe, a senior vice president at the Trust for Public Land and a former city parks commissioner. "It blows out of the water by far the previous most-expensive park that I had ever heard of, which is the High Line."

That popular green space, on an old elevated rail line, cost about $36 million per acre. And though the city has not finished Bushwick Inlet Park in Brooklyn, it spent $323 million acquiring the land, or $54 million per acre.

While the current cost figure for the second phase of Hudson Boulevard Park is simply an estimate, City Hall cited several reasons for the high price. A large chunk of the footprint is now a gaping hole over an Amtrak rail line that must remain active. In addition, the green space's entire footprint is privately owned.

Tishman Speyer agreed to help pay for the park in exchange for development rights at a planned tower. The city needs to negotiate a price with the other owners or seize the land through eminent domain and pay its appraised value.