Tremendous
Direct Cost
In Ruwais, in the UAE, cut and fill for a 1km x 1km (at a depth of 10m, including above and below the gulf bed) and all the associated costs, with cheap labor, came to about 1,000AED ($367USD) per m2 surface. Your land take appears to be 600bn sqm (3 "UKs") and about 20x the average depth.
High-level estimate of 4,404 trillion (plus 20 % contingency for such a project) comes to let's round it to $5,000 trillion USD = 5 quadrillion USD (see comments).
Associated Costs
Now you can calculate the tremendous loss to trade, tourism, coastal real estate, and the administration and maintenance of this land.
Was it worth it?