That's a staggering sum of money. It can't be waved away as rounding errors, or typical bureaucratic corruption and waste. There had better be a whole of something amazing we built in exchange for that kind of money.
Much has to do with the tracking of real property assets. This has been grossly mishandled. There are those that could of fixed the problem, but instead have been removed from thier positions. Much has been hidden in military budgets, proper accounting may expose the extent of black program spending.