This is off by 5-10x if you count all the costs (equipment, increased domestic costs, ongoing security liabilities, ...)
It also completely discounts the human cost of warfare, if such a thing is even measurable.
Definitely. Even if you just focus on the economic cost of the human costs (lost income for anyone who dies in early 20s, disinvestment in Iraq/Afghanistan by local people due to fear they'll be robbed or killed, etc.), that's higher than the direct military cost.
Can we just start listing government costs debts, etc. on a per-capita basis? Aside from the fact that we can then compare differently sized governments, it just gives a better sense of scale. When you multiply by 300M just about all numbers are mindlessly big.