It is often thought that Cognac is always more expensive, just because it is more well-known. But in reality, it is not that simple.
Some Armagnacs can be more expensive, and it is mainly a matter of how they are produced. Armagnac is made in much smaller quantities, often by families, with less industrialization. There is more human labor, less standardization, and each bottle can be a bit different. Additionally, there is the aging process: some Armagnacs age for a long time, sometimes much longer than one might imagine, which naturally increases their value.
Unlike Cognac, there is much less marketing behind it, so the price mainly reflects what is in the bottle, not the image surrounding it.
In the end, Cognac is more well-known, but Armagnac is often rarer... and sometimes, that is precisely what makes the difference.