I’m not even going to get started on the Gashmius/Ruchnius debate, as so far, what I’ve seen above concerning this inyan is pointless at best, and fallacious at worst.
There are several reasonably simple reasons why these clothing stores are expensive, some, all or none of which may apply in any given case.
The first is the above-mentioned principle of economies of scale. In cases of clothes specifically catering mainly to the frum community, there is only really demand for a limited amount of articles, meaning the cost of having them commissioned and produced is not as thinly spread, and as such leads to higher prices. Or, put more simply, making more of something means lower costs, and lower prices for the consumer, and the same is true for the reverse.
Another, similar but slightly different, factor is that many of these shops are small, high street shops. This is largely due to the fact, also mentioned above, that there are not enough customers to support a large clothing shop, which are generally on cheaper, out-of-town premises. Therefore, overheads such as rent and staff are comparatively higher. in order to turn a profit, prices are higher, and since there are fewer items sold than in large retailers, the profit on each one must be larger.
So essentially, the key reason is economies of scale, from the development and production stage right down to the store selling them.
Personally, I think the fact that there is little competition with regard to some products, like hats, streimels, sheitels, and similar niche frum products, lead to certain sellers taking advantage. Some of the markups on these products are clearly massive, for example Borsalino, where the profit margin can only be enormous. The fact that the frum consumer tends to be less discerning, and be affected by outside factors such as peer pressure, compounds this. This is especially true of the above example, but can easily be applied to other products geared towards the frum market.
However, in time, a growing community and greater recognition of some of the above issues may hopefully mitigate or indeed eradicate this.