http://gregorio.stanford.edu/holbrook/CableCosts.html says that cost of undersea cables in the '90s was in the $27k-$56k/km range, with the $27k a bit of an outlier.

Cost estimates for terrestrial cable are harder to come by, and seem depend a lot on things like rights of way (likely not relevant here as a cost), terrain (_very_ relevant), ability to get equipment to the right location, etc. But the numbers I'm seeing on various links from https://www.google.com/search?q=cost+of+laying+fiber+optic+c... are in the $20k to $40k range per kilometer (I took the actual numbers I saw and divided by 1.6) about 5-10 years ago. Figure about 1.4x smaller if you want to adjust for inflation to compare to '90s numbers.

All of which is to say that "zillion" in this case is probably a number somewhere between 1 and 4. Give or take; these are all estimates, obviously. Also not clear to me which is faster to lay, by the way, which might matter in terms of deciding which one to do.

Not to mention that the undersea cable might be shorter, offsetting some of the per-km difference in cost, since it's a lot easier to go straight.

The alcan highway is pretty long, and passes through some pretty remote regions. You'd have to run cables from the alcan down to Skagway, and then you'd have to run cables underwater to connect the various coastal communities that are on islands, or far from developed roadways. Juneau, Ketchikan, Sitka, Wrangell, Petersburg are all communites with 2,000-30,000 people, and none of those communites connect to the outside world by road.

It's easier to lay a subsea cable than to go overland in remote areas. Take a look at the subsea map of Africa as an example. In Africa the subsea links are there because it's too remote to run cables through the jungle or across the Sahara. Additionally terrestrial cables are more frequently cut than subsea cables. In India terrestrial fibers are cut routinely every day to the point that they have to allocate extra repair margin for the links.