In spite of its title, McArdle's Guardian essay isn't about economics at all, at least the economics of the market place. The costs she talks about are psychological and relate to the literary merit of the piece and inconsistencies in the plot.

Actually, this is what has always bothered me about fantasy, the hard stuff is too easy (incredible powers invoke fantastic results) while the easy stuff is just as hard as it is for the rest of us (as she points out, the Weasleys can't even conjour up a pair of new shoes).
It's like the old Flash Gordon, they fly around in spaceships, and when they get there they fight with swords.

As for your latinum example, the the effect of and difficulty of reproducing a medium of exchange (certainly an issue in real economics) is a testable proposition with real world consequences. Does getting off a gold standard have economic implications? That's a matter of debate today. Does greatly increasing the availability of gold in a short time have economic consequences? It certainly did for 16th century Spain when it started looting the Aztec and Inca treasure. Suddenly finding a huge new pile of gold is the economic equivalent of learning how to make a perfect counterfeit of it.