This is an aside, but...
It's worth pointing out that the Pentagon was built in IIRC the 1940s, and for that reason alone was far less expensive than most modern buildings - when you AC consists of "Open a window.", you're going to save a lot compared to more modern centralized HVAC designs. The entire building was essentially gutted and refurbished in more recent years to address this and many other modernization issues, making the initial price pretty much meaningless as far as direct comparison goes.
Also, I can't even imagine the price DoD would want to spend on a new headquarters located anywhere close to DC. However, given the processes and requirements in place today, I'm guessing the price would dwarf that of the new embassy in London, and would likely be what many would call a "boondoggle"

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Not an attack on you, but I think it's worth pointing these matters out if you're going to make a comparison.