I believe the President effectively drove the price down. If Denmark turned down what was probably a better offer, China can offer lower and know they have Denmark behind a barrel.
As I understand it, Danish law allows Greenland to declare itself independent if a majority of its residents vote in favor of it. There are about 60K Greenlanders, and the sentiment is overwhelmingly in favor of separation, except that they realize that they cannot support their current standard of living if they did. All Trump needs to do is to convince the 60K living in Greenland, and not the Danish prime minister. If the U.S. offered a hefty one-time payment to all Greenlanders, along with a U.S. passport and citizenship, I expect it wouldn’t be too hard a sell.