Original Cost: $375 million
Inflation Adjusted Cost: $7.9 billion
The Panama Canal was the single most expensive construction project in US history to that time. Amazingly, unlike any other such project on record, the American canal had cost less in dollars than estimated, with the final figure some $23 million below the original estimate, in spite of landslides and a design change to a wider canal. The first ship sailed through in January, 1914.