I'd think the question would be are *you* satisfied with the run time you are getting with the adaptor or not. If not, it may be worthwhile paying a little more for the battery if you think it will genuinely improve your vaping experience. If you are happy enough and get good battery life with the adaptor, well that is what it is for.
I think they are pricier because they are larger and heavier. One of my mods came with the larger battery (it was a win, the Capo) and I was surprised at how heavy it was, even with the mod exterior being plastic. I'm pretty happy with the batteries I have now, and am not in a rush to buy mods with two of that size battery or anything, although I can see how they might be useful to some. For me, I just don't need the extra weight in comparison to 18650 battery mods.
Best of luck deciding.
Anna