The Louisiana Purchase gave the USA about 800,000 square miles of territory for $15,000,000. The other big land deal was the purchase of Alaska (a little less than 600,000 square miles) for $7,200,000. The Louisiana deal seems to be the be the biggest in terms of area. It also works out better in terms of area of land per dollar. And it more or less doubled the area of the country at the time.