In 2016, the Government Accountability Office estimated the total cost of replacing the Air Force One fleet
to be $3.2 billion, including costs related to research and development, procurement, and military construction spending.
Two years later, in April 2018, the GAO estimated this cost to have reached $3.7 billion, though this figure does not include military construction spending. The GAO told
Newsweek military construction spending for 2018 was $314 million, giving a total estimate of just over $4 billion.
The GAO report noted that since the contract had not been awarded at the time, there was no baseline from which to gauge cost performance of the project, making it difficult to keep track of how much money was being spent and would be spent.
According to the
Air Force budget for 2019, the total military construction cost relating to replacing Air Force One plane was $254 million. Combining the GAO's latest cost estimate and this figure gives a new total estimate of more than $3.9 billion—bang on Trump’s new deal.
So how, exactly, the White House reached a number of $1.5 billion in savings remains unclear.
There is, however, a mathematical error easily made in using the GAO’s 2016 estimates that would lead to a total of around $5.4 billion, making Trump’s deal of a $3.9 billion fixed cost with Boeing appear like a $1.5 billion savings.