I was in West Germany during and after reunification. It definitely turned out to be much more difficult and expensive than most people expected. Considering that East Germany was much wealthier than North Korea I expect building them up will be even more difficult. Some people will benefit greatly but overall it will probably a massive expense for South Korea without any benefit for most citizens.
It was a powerhouse before. From my memory the mood in the West got worse. There were tax raises and a lot of uncertainty when it became clear that it wasn't as easy as initially planned to get the East going economically. There was a substantial number of people on both sides who wanted the wall back.
I am not saying the reunification wasn't worth it but it was much more difficult than people may think.
For what it's worth, the West German government did honor East German marks 1-to-1, even though there was a gigantic disparity in value of both currencies. That alone was an enormous cost of the unification.
A lot of people thought that was a huge mistake. After the wall fell there was also a democratic East Germany for 2 years (?). I think a lot of people would have been OK with keeping them separate for a while but chancellor Kohl wanted to put his name on history and forced the reunification.
