At $15 million, the Louisiana Purchase might sound like a relatively small real estate deal in today’s market. But it nearly doubled the size of the United States when it was signed on April 30, 1803. According to the National Archives: • In this transaction with France, the United States purchased 828,000 square miles of land west of the Mississippi River for $15 million. • That amounts to roughly four cents an acre. • The lands acquired stretched from the Mississippi River to the Rocky Mountains and from the Gulf of Mexico to the Canadian border.