My best (stupid) guess is weapons. US gov has a ton of weapons. Already sells tons of weapons. Now the prices will always remain stable. Other countries would love to know that 30 million usd would always be able to buy an f15. Other countries declaring war will increase the value of the USD, as buying weapons from us government will decrease amount of money in circulation.
God, imagine a country going into irrevocable debt and having to actually export people.
Also, currency would fluctuate wildly as people immigrate/emmigrate.
Do you count tourists? Illegal immingrants? Legal, but not sovreign, citizens (or whatever the term is for someone working in a country on a visa)?
What if people fudge their measurements?
Does put a direct price on war, i guess.