I’ve read the paper through once. All goes well until the authors count the interest payments on the debt as an extra cost. I say count the expenditure once and do not adjust for how it is financed.
Starting on p.14, the authors consider macroeconomic costs; if not for the war our rate of growth could have been higher. In particular these costs stem from higher oil prices, higher defense expenditures, and increased insecurity. This part of the paper is highly speculative. Our economy has done fine, and it is not clear that the residual problems are due to the war.
Before counting macro costs, the authors estimate the costs of the war to run about $700 billion and $1 trillion dollars. This appears well-founded but the higher estimates (about twice that) do not. Note, of course, that none of this considers the costs (and benefits) to the Iraqis. Returning to the American side, the authors do adjust for the costs of running the pre-war no-fly zones, but they do not attempt to estimate what other costs would have been incurred in the absence of invasion.
Comments are open, especially for those who have read at least part of the paper. Analysis is welcome, but general or polemic opinions on the war will be deleted.
Addendum: Anthony Battey directs my attention to another paper on this topic.