Trump believes Greenland is for sale
bbc.com
bbc.com
I'm not seeing any mention of "buying" here. Given his comments on the Panama canal, I think you should assume he has other options in mind for acquiring Greenland.
https://www.thetruesize.com/#?borders=1~!MTQ5ODUwMjI.OTMwODU...
Clicking the outline will let you drag it around, and when it's selected the compass-rose in the corner can be twisted to change the orientation.
If you own just a little land and you get $1M can be a good deal but if you own a lot, then you will expect premium on top of what its worth and $1M not going to cut it. If everyone gets accordingly to what they own, then those who will receive non-life changing amount wouldn't accept the deal.
So the true price would be something like $1M each + current value of their property + premium.
Then you can own all the private land on Greenland if someone stubborn doesn't refuse to sell. Then what do you do about the rest of the land? Pay Denmark? How much? Start a war with Denmark? Maybe some kind of other deals can be possible but $56B isn't going to cut it.