Back of the envelope calculation based on the needed size(1 million square miles) and the proposed cost of a pilot project(10-20 million USD for 100square feet)

Back of the envelope calculation based on the needed size(1 million square miles) and the proposed cost of a pilot project(10-20 million USD for 100square feet) suggests a total price(~3e18 USD) 4 orders of magnitude more than world GDP(~1e14 USD). Now to be fair, that's without factoring in economies of scale or anything, but we also have to factor in the fact that literally all of world GDP can't go into just one project.