At first this maybe sounds like a joke or almost impossible thing to do, but when you think about it, it is great idea as United States did similar things before, buying territories from other countries.
In 1805 United States bought Lousiana for $15M from France
In 1867 United States bought Alaska for $7.2M from Russia
In 1917 United States bought Virgin Islands from Denmark for $25 M
In 1946 Harry Truman tried buying Greenland, but that time Denmark didn't accept the offer.
So as we can see, this is perfectly reasonable and smart move by Trump, if he manages to pull this off he will be remembered as one of the most successful presidents in history.
The USA buying Alaska from the Russians is what i will call the best deal ever made in the entire Earth and maybe Trump sees somethig with Greenland which i don't but with his claim that the Island is costing Denmark around $700M in subsidies is something that may change the mind of the Danish government in selling the territory.