Jefferson believed that the united states need to purchase Lousiana eventhough the cost might be a little bit expensive.
In 1801, there was a threat coming from The french that if United States did not buy New Orleans from them, they might started a war.
United States couldn't bear to face another war not long after the revolutionary, so Thomas Jefferson went to purchase in order to talk about the Deal.
He returned with a compromise to buy Louisiana instead of New Orelans from the french because it benefits united states the most.