The Chinese like to think they take the long view of such things. Certainly it would cost the Skors a fortune, but they would have little choice but to pay for reunification, it is a national prerogative.
And once it was done, from the Chinese point of view, they would finally be rid of an annoying threat on their border than has troubled them for almost 60 years.
Winning the friendship and huge markets of all of Korea and only losing a pestiferous, dishonorable, thieving and treacherous Kim sounds like a really good deal for them.
And from the US point of view, Korea would be unoffensive, maintaining a form of democratic government acceptable to both the US and China, and also keeping trade ties with the rest of the world. And if Korea is happy with China, far be it from us to stand in their way.