Now those numbers are actually pretty reasonable. If Nobody ever dies, there's 10 million in damages each time an event like this occurs (around once every 10 years now), and with climate change we can expect the intensity + regularity to increase over time, then 1 billion is likely a good price to pay for solving the problem.
Now with mitigations like those that would be used here (like the storm water systems used in Tokyo), the sooner you implement them, the easier/cheaper they'll be to implement/maintain long term.
At the end of the day the best time to start adding the mitigations is now and expand later as necessary rather than wait until the cost is high enough and go into a mad dash to complete them before the next disaster.
Edit: For context, the Metropolitan Area Outer Underground Discharge Channel in Tokyo cost 2 Billion USD to build and is able to easily handle this volume of floodwater.
I was the original poster suggesting the Tokyo System - but I did the math and I was wrong.
The total underground capacity of the Tokyo consists of the Cans, (250K cubic meters), Cistern (248K cubic Meters) and Tunnels (575K cubic meters) - for a total underground capacity of 1.07 million cubic meters.
New York City was hit by 35 Billion gallons of rain during the flash flood, which = 132 M cubic meters. The Tokyo System has .38% of the capacity of the rainstorm, and is capable of draining at 200 cubic meters/second which would take 7-8 days to drain all that water.
What happened in New York was epic - and no underground system imaginable would have been able to handle it. The only solution to this type of problem occurring in the future is going to be around things like zoning (don't build where it's going to flood), landscape (lots more greenery), architecture (lots more void decks, first floor resilience to flooding, no basement suites, etc...)
Now with mitigations like those that would be used here (like the storm water systems used in Tokyo), the sooner you implement them, the easier/cheaper they'll be to implement/maintain long term.
At the end of the day the best time to start adding the mitigations is now and expand later as necessary rather than wait until the cost is high enough and go into a mad dash to complete them before the next disaster.
Edit: For context, the Metropolitan Area Outer Underground Discharge Channel in Tokyo cost 2 Billion USD to build and is able to easily handle this volume of floodwater.