Final answer:
The United States paid $15 million to purchase the Louisiana Territory from France in 1803, which was a significant acquisition that doubled the nation's size.
Explanation
The United States completed the Louisiana Purchase in 1803, acquiring a massive territory that effectively doubled the size of the nation. President Thomas Jefferson recognized the value of the land and the strategic importance of controlling the Mississippi River and New Orleans. Despite his initial reservations about the constitutional authority to make such a purchase, the desire for an agrarian society and the potential for westward expansion were compelling reasons for Jefferson to proceed. The U.S., represented by Robert Livingston and James Monroe, negotiated with France, and Napoleon, eager to sell, agreed to the transaction. The final treaty was signed on April 30, 1803, and the U.S. agreed to pay a total of $15 million, which was a bargain price for the vast swathe of land they acquired.