So here's the thing with Greenland. It's mainly indigenous people living there (inuits), a bit under 60k. 40% of their working population (whole working population: 25k) work in administrative jobs, which are more or less subsidized by Denmark. Denmark subsidizes Greenland to the tune of half a billion euro annually. The rest work in fisheries, hunting, whaling, animal husbandry, and things like that.
Very few work in mining, because there's little infrastructure in place to engage in mining. And while it would take a huge sum of money to start mining there, who knows if the people there are interested in working with mines, or if they would even allow it. If you've ever lived in a country with indigenous people, it is a regulatory nightmare to just mine on places they claim to own. I live in Norway, and a minority of our indigenous people (Sámi people) work with reindeer herding. It is a very subsidized sector, and the regulatory red-tape tied to reindeer areas have stopped many projects related to energy and minerals.
My point is that no mater how lucrative the new alternative is, plenty of people will straight up reject it on basis of tradition. Some things you simply can't put a price tag on.
If US rolls up and wants to buy Greenland, from Greenlanders, they also need to convince them to pretty much give up all their land, their way of living, and to take care of them per the Nordic model, for the rest of their lives. The average Greenlander earns something equivalent to $50k / year.
And how much would US be willing to pay? What if the citizens of Greenland have a vote, and say - OK, we'll do, but only if US can pay $50m / citizen ($3 Tn), or some arbitrarily large sum. Or if they say, nah, we're fine as it is. No money in the world will be enough?
Point is, the US is the party that wants Greenland. Greenland knows this. They could chose to say fuck off, we don't want you, or they could demand as much as they want.
If US then decides to take Greenland by force, it would set off things that will cost US much more than what Greenland will ever be worth.