Put simply "stuff". Take a look at Fred Brooks, mythical man month if you haven't ready it already. The projects that cost $250M will have over 1,000 engineers

Put simply "stuff". Take a look at Fred Brooks, mythical man month if you haven't ready it already. The projects that cost $250M will have over 1,000 engineers involved. Running a tightly coupled engineering project of that size is impossible in my opinion, thus the cost. Figure that 50% of the budget goes to communication overhead, 25% to management overhead, and you are left with 25% of true engineering costs.