Jim at Objectionable Content has some comments regarding my piece on the price of civil order. He calculates it per-household and per-adult. He also calculates per-household defense spending as $253/month.
I did not calculate defense spending in the original, even though I thought about doing so, for a good reason. Most of what the justice system produces is private goods. If you don't subscribe to a company A, it doesn't protect you. Most of what we think of as "defense" spending is a public good. Public goods are underproduced, or not produced by for-profit producers. Defense being a very broad-scale public good, in anarchy I expect it will not be produced on the market. Since defense is valuable, it will be produced in other ways - primarily, I think, charity. But the total spent would almost certainly be far less than the USA currently spends. And the cost is likely to be spread much less evenly than that for protection (which everyone needs all the time).
Economics lesson for those confused by "public good" and "private good": a private good is a good which you can exclude others from (and because you can exclude, you can make money!). Most normal goods are like this. If I eat the apple, you can't also eat it. A public good is a good that, once produced, benefits a set of people who cannot be excluded from enjoying it. Think of a NPR radio broadcast: regardless of whether you cough up money at the begathon, you can still listen. Economic theory tells us that private goods will be produced in the correct amount in a market, but that public goods will be underproduced.