To my knowledge, Denmark has never even hinted that Greenland is for sale, or, if it were for sale, how much they would want for it -- $billions? $trillions? Anyone's guess.
dodger: "...whereas it does cost Denmark far more - proportionately.
If, as you say, it is as a fixed sum 10% of their military budget it would be a vastly lesser proportion of ours"
Right, the cost of Denmark subsidies to Greenland is circa 10% of their defense budget, which for the US is nearly $100 billion per year, proportionately, a considerable sum.
So, perhaps the Danes would be happy to shed those costs, who knows?
dodger: "Huge strategic upside at marginal cost."
I'd agree with the upside potential, perhaps not so much with your "marginal cost".
Greenlanders rather than Danes may be the deciding party.
The Danish annual support is a fixed amount. This amount may be10% of what they spend on defense / aid, but a far smaller percent of the US budget (i.e., we would spend nowhere close to 10% of OUR budget).
A bargain if we can …