for less than the cost of Sandy, NYC could have built themselves a surge barrier across the Verrazano narrows.And that would help the Atlantic coastline of Staten Island, Brooklyn, and Queens exactly how? Those being the sites most devastated by the storm surge. 
For those unfamiliar with NYC, the Verrazzo Narrows Bridge connects Staten Island and Brooklyn at Interstate 278 on the above map.
It would certainly not protect Staten Island, etc but those are cheap to rebuild in comparison with Manhattan, A barrier to protect Manhattan would probably make the surge worse elsewhere.