The developer gets millions in tax breaks per year, per building. Average of $114,000 per building per year with the inclusion of the low-cost units versus $4,400,000 without such exemptions.
"...five of the luxury firms towers cost the city $21.8 million in tax revenue in their first year alone.
Together, the buildings paid just $567,337 in annual taxes. Without the 421a program, they would have paid the city $22 million, according to appraisal firm Miller Samuel Inc."
http://nypost.com/2013/08/18/upper-west-side-condo-has-separate-entrances-for-rich-and-poor/