My expectation on the cost being far too low, was taking into account that South Korea is an excellent nation and that they may be better at fixing eg North Korea than a lot of other countries in their situation might.
Most countries in the world would simply not be able to fix North Korea, or pay for it. It would be a perpetual disaster, with the North falling back apart, languishing from quasi-dictatorship to quasi-dictatorship.
The difference between South Korea lifting itself up over 20 or 30 years, and what's going to happen with North Korea, is responsibility and imbalance. South Korea is going to end up assuming immense responsibility for the well-being of the people in the North, rather than the North existing as its own country; and the South will be massively rich by comparison, that imbalance will draw out very large sums of subsidization in the process of bringing the North up to speed to the South's standards --- there was no such scenario with the South when it originally climbed up out of poverty, no stark comparison amongst their own people essentially. It will be viewed as very unacceptable to have the people of North Korea with a per capita GDP of $10,000 after 20 years, while the South perhaps has a $50,000 per capita GDP at that time. Many in the North will not want to stay there in those conditions, and by moving to the South they will generate a significant new financial burden. Either way, it's going to be very expensive.