Not sure how financially doable it would be to buy the rights. Though, I cannot say if the entertainment industry in Korea is setup the same way. However in the US, this is roughly how things work:
An entertainment company will buy an "option" to the rights of book, novel, graphic arts, etc... for roughly 12-18 month span. The cost of the option varies anywhere from $500 to 10% of the final "rights price" depending upon the popularity/demand for the property.
Now movies and TV shows use different formulas, (I only know of examples for movies, so this is not an exact match) regarding actual "rights price" when the company is ready to make the movie/tv show. In the case of a movie the final price will be in the hundreds of thousands (US dollars) for lesser known works. In the case of best sellers it will be in the millions (US dollars) though usually does not exceed 3% of a movie budget (a blockbuster Hollywood movie will have a budget above $100 million).
Getting back to Goong season 2, since the "rights holder" will know that the 1st Goong was successful. They will obviously ask for top dollar for the "rights" purchase.
NOTE: All of the above is in US Dollars, there is a 1100 Won to 1 US dollar (roughly). Also, since I do not live in Korea I am unsure what the average costs/budget of the Korean entertainment industry are. I would assume they are smaller than the US because the differences in the respective populations and size of economies.