“ The cost of building a so-called North Sea Enclosure Dyke, estimated at between €250bn and £500bn, amounts to barely 0.1% of the combined GDP of all the countries that would be protected by it, they calculate.”

That would mean those countries GDP is estimated at 250 to 500 trillion pounds or 320 to 645 trillion dollars

France 2.6 trillion, england 2.75 trillion, Scotland 202 billion, Netherlands 826 billion. Or about 6.4 trillion dollars per Wikipedia.