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.

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?