How much did the Louisiana purchase cost back then?
The cost was 15 million US Dollars or 60 million French francs. Because this was paid in installments, the total including interest was $23,213,568. Jefferson originally sent Monroe and Livingston with $10 million to purchase New Orleans, but Napoleon offered them the whole of Louisiana for only $15 million, so they could not refuse.
The USA grew much much larger with the Louisiana purchase because what you know of as the 'Plains states' were all part of that purchase. Practically all the land that is part of the USA from Saint Louis through the Rockies was all purchased at that time. More The size of the United States doubled with the Louisiana Purchase. There is a link below for more information.
According to the Bureau of Land Management, the Louisiana Purchase was 828,000 square miles of French territory in 1803. The cost was about $15 million (3 cents per acre). The land purchased was all of the land in the U.S.A west of the Mississippi. 828,800 square miles in 1803 for a total of 15 million dollars ($219 million in 2010 $)