A secret wartime project spent $2 billion to build one weapon

The Manhattan Project, the United States' program to build the first atomic bomb, ran from 1942 to 1946 under General Leslie Groves, with J. Robert Oppenheimer directing the weapons lab at Los Alamos. At its peak it employed nearly 130,000 people across sites in Oak Ridge, Los Alamos and Hanford, most of whom had no idea what they were building. It cost roughly $2 billion at the time — about $28 billion today.

— Wikipedia contributors, Manhattan Project — United States, 1942–1946