South Korea is a rich nation. It would surely tax them for years, but at the same time it would bring them lots of work as well. China would probably help a bunch as well, being right next to North Korea and having more money than they can imagine use for (since they can't imagine using it for the benefit of their own people). Above all, North Koreans are still Koreans. It's not like they would be getting millions of foreigners suddenly in, like could be the case if North Korea had been occupied by some third country all these years.

Still, it would take a miracle for North Korea to open. It's not up to the leader alone; there's a whole power structure including the military brass that is very opposed to losing their privileges.