"We have 540 billionaires in the U.S. If every one contributed $1 million like Drumpf did, it could make a HUGE difference."
$540 M is a mere drop in the bucket towards trying to sort out this mess.
Over and above the obvious personal losses, the real problem is the environmental impact from the chemical cesspool that surrounds the city...before the storm even hit there were 13 Superfund sites there.

I think it's also necessary to severely evaluate the rebuilding process because this "500 year flood" is no longer an outlier and weather events like this will become more and more common. Rebuilding on a giant floodplain like Houston really makes no sense. I've only heard a few peeps so far, but it's certain that insurance companies are going to reevaluate their flood insurance provisions and requirements to reflect the current situation.