Proper oversight and project leadership (as in, it goes beyond "call McKinsey and have them send over a couple of seniors") can save costs at many parts of the process:
- catch politicians' bullshit before planning (plans that, say, lead directly through a nature reserve) or during construction (e.g. a new politician comes in and wishes some random shit to change just to be able to brag "I did something") and intervene
- check bids on tenders if they are actually plausible or if they are so far under a realistic budget that there will be massive problems later on (e.g. the company threatening insolvency to demand re-negotiated higher rates)
- check impediments if they are actually legitimate or if they are overblown and resolve them
- keep vendors accountable to milestones
You can forget about the first two points, which are the most critical, for external consultants.