It was also insanely expensive for west germany (though not as much as here), but it was their goal from the start.
>Germany agreed to pay about 55 billion deutschmarks to the Soviet Union in gifts and loans, the equivalent of eight days of the West German GDP.[1]
Which if applied to US numbers 2016 GDP would be about 0.5 trillion dollars. The same source said the Germans would have paid 100 billion marks or roughly a little under 2 trillion.
[1]:https://en.wikipedia.org/wiki/German_reunification
Quickly googling numbers, it get between 100 billion euros and a trillion dollars[2]
[2]:https://www.quora.com/What-would-it-take-for-North-and-South...
The economic ration between the two countries are estimated to be not the 1:3 east and west germany was but more like 1:20 and the norks has half the numbers SK has, whereas there were 1 east german for every 2 west germans.
However that also does not take into account that a united Korea would be a huge economic powerhouse[3] (with a fully developed north, which will obviously take some time), Goldman Sacks estimate that it could have an economy that is bigger than any other G7 country, except the US.
Given that interest rates are super low right now, it might not be a bad idea to get some long interest loans for a unification.
[3]:http://koreatimes.co.kr/www/news/biz/2009/09/123_52202.html