Why did an imperialistic country like France (at that time) sell a territory so large for so cheap? Was it an important region at the time? I heard that the port of New Orleans was particularly valuable. Even if at that time maybe it wasn't so useful, look at it now!
The sale of Louisiana was a FIRE SALE for France, and specifically, its self-appointed ruler, Napoleon Bonaparte, for these reasons:
1) France had gotten "burned" with her earlier misadventures in North America. The French and Indian war cost her Canada and all of her other possessions on the east bank of the Mississippi. (She had managed to save "Louisiana" by "parking" it with Spain in 1763, then taking it back in 1803.) France was being "burned" again by a(n ultimately successful) slave revolt in Haiti that cost the life of Napoleon's brother in law.
2) Bonaparte was engaged in a war with Britain, and needed money for that war. If he didn't win it, there would be no "tomorrow," at least for him. (He spent is last days as a British prisoner on St. Helena island.)
3) New Orleans was far more valuable to the United States (which had settled half a continent) than to France, which had few settlements nearby. It also threatened to be a focal point of Franco-American hostilities, something Napoleon did not want. Instead, the sale generated "goodwill" that caused America to favor French interests over British interests, ultimately ending up with France and the United States fighting a common enemy beginning in 1812.
Here's what Jefferson wrote to Robert Livingston about the third point:
An important factor was the health of Napoleon's troops. I forget the numbers but Napoleon sent around 50,000 troops out to Haiti and they were devastated by yellow fever. New Orleans was also a hot-bed of yellow fever at the time.