Nope, tradable goods don't just become "more expensive" because of currency shifts. The prices of tradables are set in a global market and tied together by cross-trading (arbitrage) considerations. What happens when supplying a desirable product is that the industry seeks to expand and that might increase costs overall. The Dutch Disease is an extreme version of this, in which having a single dominating industry sector leads to e.g. bad governance, etc. But these are real effects that are unrelated to currency shifts.
It sounds like you might be talking about commodities, but that's not every market. For goods manufactured and largely consumed in one country, currency shifts do quite mechanically make foreign prices cheaper or more expensive, assuming they are still sold for the same price domestically.
(Though, the price might not change right away depending on long-term contracts and whether the manufacturer decides to take profits or accept a lower profit margin.)
The important question is: Does Narnia have the money, or is it willing to spend, double the amount on your product? Maybe they will just buy half, because their budget is fixed. That's the point you are missing.
....I fail to understand the relevance of what you're saying.
A desirable good is still less value/money if the money cost increases. That's what we've been discussing in this thread. Now you're changing other variables?
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.
