Other examples might help, especially if they are more recent than the Louisiana Purchase.
Wikipedia has a list of United States territorial acquisitions, which includes the following paid ones:
- Louisiana Purchase: "$15 million (equivalent to $251 million in present-day terms)."
- Mexican Cession: "$15 million (equivalent to $403 million in present-day terms) and agreed to pay claims made by American citizens against Mexico which amounted to more than $3 million (equivalent to $81 million today)."
- Gadsden Purchase: "$10 million (equivalent to $301 million in present-day terms)".
- Alaska: "$7.2 million (2 cents per acre) on March 30, 1867 (equivalent to $129 million in present-day terms)".
- Spanish colonies (Puerto Rico, Guam, Philippines): "$20 million, equivalent to $602 million in present-day terms".
The most expensive was the most recent. That included the Philippines, which are no longer considered US territory. That also ended the Spanish-American War, so the money may have been for more than just the transferred territory.
The Gadsden Purchase was one of the two smallest.
Three of the acquisitions were larger than Montana. The most expensive was less than a billion dollars even in modern terms. From a historical basis, none of them have a territorial value that is even a thousandth of a trillion.
Beyond all this, Montana seems not the best choice. Alaska is contiguous with Canada but not any other US states. But Montana and Alaska both voted for the less Canadian-like politician. Their populations would not fit Canada.
Some places that vote for Democrats, who are more like Canadian politicians, include
- New Hampshire.
- Rhode Island.
Some cities that are relatively close to the Canadian border (or to the New England border):
- New York City.
If we add New Jersey (connected to New York City), that would also allow
- Northern Delaware.
- Washington, DC.
Puerto Rico could go with them as well.
That population is much more consistent with Canada in their political views. Such a swap would give them a government more in line with their views.
This would be less land than Montana, but more people and economic activity. Something like 35 million population. Perhaps $2.5 trillion GDP.
More importantly than a simple money transfer, presumably Canada would pick up the obligations. Such obligations would include a couple trillion in debt but also trillions more in Social Security and Medicare obligations.
But as already mentioned, this wouldn't really work as a sale. It would make more sense as an independence vote.