You were making a point about what happens if the exchange rate changes and everything else stays the same. That's not particularly relevant to the real world because if the exchange rate of a freely traded currency significantly changes, then all the other relevant factors also are very much not the same; it has a lot of interlinked consequences, and it can't happen without a significant economic reason pushing for that change (artificially fixed currencies have their own intricacies).

The direct, immediate price effect is exactly as you describe but at least in the medium term it's also affected to a significant (and perhaps even larger) extent by (a) the major macroeconomic factors that caused the Narnian gold pieces to devaluate w.r.t USD; (b) the changes to the income and global purchasing power of Narnian people caused by that devaluation; (c) the long-term changes to the widget trade and competition; and a bunch of other ripple effects; so the effective price to them won't double; it might increase somewhat (but much less than double), and it might even decrease within a reasonable time if that devaluation fixes some underlying structural economic problem and thus their income; It's quite likely that the devaluation was caused by systematic factors that screwed (temporarily) their productivity and income; so large changes in exchange rate don't happen just because.