The Trump negotiated Air Force One contract should be the standard. Contractor agrees to deliver X by this date for this amount of money.
The only cost overruns are if the government's specs change.
https://www.politico.com/news/...
Trump got personally involved in the negotiations for the replacement aircraft. In February 2017, Trump said the Air Force was “close to signing a $4.2 billion deal” and that “we got that price down by over $1 billion.”
“President Trump negotiated a good deal on behalf of the American people,” Boeing said in a tweet after the contract was finalized.
Instead of negotiating a contract that protected the company from changes in supply costs and other factors, Boeing and Trump settled on a fixed-price contract that forces the company to carry the risk and not charge the government.