I have spent 5 dollars of a 15 dollar project. Completing this project would (probably) net me a return greater than 15 dollars. It would not make sense for me to spend 5 dollars and not to complete the project because even if it is doomed to fail I would never know because I never finished it.
Sunk costs mean nothing when a project needs more government funding. If the government feels like a project's budget is going way out of control, they'll cancel it, regardless of whether money has already been spent.
Notable case: The Superconducting Super Collider. Approved by Congress in the 80s, the quoted cost was $4.4 billion. In 1993, they revised the cost up to $12 billion, after $2 billion had already been spent, and the SSC was canceled.
