When you start talking truly cataclysmic events, the value of the physical infrastructure at risk will be dwarfed by the human lives. In 2016, the assessed value of all real estate in Manhattan crossed the $1 trillion threshold (which of course leaves out the bridges, the subways, the personal property...) At a value of $9.1 million / life, there are $77.7 trillion worth of humans in New York City.

If there's a natural disaster which would wipe out the population of New York once every 5000 years, we should be willing to spend $15 billion per year to prevent it.