There are a few casualty estimates out there of the cost of a war with North Korea. A couple of these casualty estimates are summarized in this article: https://www.yahoo.com/news/u-must-invade-north-korea-091003273.html
1. As many as 2.1 million could die if nuclear detonations occurred over Seoul and Tokyo (source: website 38 North, October 2017).
2. As many as 300,000 could die in the first few days of a conflict between North Korea and the U.S. even without the use of nuclear weapons (source: Congressional Research Service, 27 October 2017: https://fas.org/sgp/crs/nuke/R44994.pdf)
The Dupuy Institute has not done any casualty estimates or analysis of a war with North Korea, nor are we planning to at this juncture. We have done a few casualty estimates in the past: