Short answers, pulled from the story.
The United States paid fifteen million dollars for the Louisiana Purchase, equivalent to less than three cents per acre. The U.S. Treasury did not hold this sum outright; the government borrowed it from British and Dutch banks at six percent annual interest. With interest, the total cost of the bonds came to $23,313,567.73, paid off in full by 1823.
Napoleon sold Louisiana because his plan to rebuild a French colonial empire in North America had collapsed. The military expedition he sent to retake Saint-Domingue was destroyed by resistance and disease, and renewed war with Britain made Louisiana militarily indefensible. Without Saint-Domingue's revenues, the territory had no strategic value, and Napoleon chose to sell it rather than lose it to British or American forces.
The Louisiana Purchase covered roughly 828,000 square miles in the central United States, including the entirety of present-day Arkansas, Missouri, Iowa, Oklahoma, Kansas, and Nebraska, large portions of North and South Dakota, the parts of Montana, Wyoming, and Colorado east of the Continental Divide, and portions of Minnesota, New Mexico, Texas, and Louisiana. It also included small sections of what are now the Canadian provinces of Alberta and Saskatchewan.
The purchase's constitutionality was disputed at the time. Jefferson himself considered a constitutional amendment to authorize it. Secretary of State James Madison and Treasury Secretary Albert Gallatin argued that the president's treaty-making power under Article II, Section 2 was sufficient authority. The Senate ratified the treaty on the 20th of October 1803 by a vote of 24 to seven, and the House authorized funding despite a close vote of 59 to 57 on a motion to deny it.
The Louisiana Purchase Treaty was signed on the 30th of April 1803 at the Hotel Tubeuf in Paris. The signers were Robert Livingston and James Monroe on behalf of the United States, and Francois Barbe-Marbois representing France.
The Louisiana Purchase was negotiated without consulting the Native American tribes who inhabited most of the territory. What the U.S. actually acquired from France was the preemptive right to obtain Indian lands by treaty or conquest, not direct ownership of lands the tribes occupied. The decades that followed brought forced removals culminating in the Trail of Tears. A 2017 estimate found that U.S. payments to Native nations for Louisiana Purchase lands through 2012 totaled around 2.6 billion dollars, roughly 27 times the fifteen million paid to France.