Does it even make sense to divorce the price of land from the buildings? Developed farmland also has a different value from virgin land, but the difference is ignored.
Manhattan's land would not be worth 1.7T$ if it were just farmland.
Of course you can approximate the price: For each plot of land you consider the value under the counterfactual where any buildings on it weren't there but all the neighborhood surroundings were unchanged. The sum across all the plots in the city is the total land price.