The WSJ has a typically well researched article on what the aftermath of a N. Korean regime implosion might be and why the Chinese / S. Koreans fear it -

As the United Nations Security Council mulls sanctions against North Korea, it must consider one question that could affect the stability of North Asia for years to come: Would the collapse of Kim Jong Il's government prove more dangerous than leaving him in charge of a nuclear-armed state?

...Another possibility is civil war, with factions within the army and other parts of the elite struggling for control. Such a scenario could prompt China, South Korea or other players to intervene, either to back favored successors or to attempt to quell violence.

U.S. military experts say they expect Chinese troops would move into North Korea to establish a buffer zone and prevent refugee flows across the border.

...A collapse of the Kim regime would also likely spark economic dislocation. South Korean government planners predict more than two million North Koreans, out of a population of 23 million, would flee south across the demilitarized zone, overwhelming social services. The planners say financial markets would swoon and inflation would jump. Crisis-management costs alone could total $6.5 billion.

Marcus Noland, a Korea expert at the Institute for International Economics in Washington, estimates the cost of unifying North and South Korea would be about $600 billion over the first decade the two states merged.

Now I'm a fan of well articulated, well researched contingency plans (although that doesn't mean that when push comes to shove, someone out there will accuse whomever's in office of "not planning for the Peace").   And, for that matter, I'm a numbers / cost-benefit type of guy and in most circumstances, I really do take these numbers in.  No one would ever accuse me of being a softy and tossing economic considerations out the window altogether.

BUT, I'm still pretty surprised that a cost of $26K/North Korean head ($600B / 23M NK's) or $14K/South Korean head ($600B / 48M SK's) over TEN YEARS is considered too steep.   I suppose that in many corners of the world, the unseen cost of 23M North Koreans silently suffering is far worse than dealing with the aftermath of lancing the boil.