Indestructible Hoya HD Filter
As far as know, there is no good reason for them to be (usually) double the price.
The manufacturing costs and techniques, and materials, are essentially the same, and that both items are low volume productions that incur the same volume/price handicap.
But it is not just specialist filters that suffer from this 'mark-up', it is the same with 2" star diagonals.
The bottom line is that the prices are so high because of 'mark-ups' from manufacturers and retailers, and from people that unquestionly pay such prices.
Luckily, as you may know, you can still use the cheaper 1.25" filters and eyepieces with an adaptor that fits into 2" holders. And because the light pollution filters are not so effective at very low magnifications the there is no need to use those filters with your 2" eyepieces.
Just read your excellent review on LPR filters, but why are the 2" filters so expensive?