Adding complexity naturally adds time and cost. for the fish knife, the bolster is layered, forged, shaped bronze that on its own took over half a day to make. The blade is very long and very thin, which takes much longer to forge and grind correctly, and and a general rule, a saya takes around 10 - 15 hours to make and finish depending on size, so costs £150-200 + material cost.
This fish filleting knife with saya and specialist damascus steel, took around 50 hours to make including a lot of design time. The materials cost around £200, so a highly specialised unusual knife like this would cost £800-900 and up. The stranger the concept, the longer it will take to design and get right. I love doing them! But they take more time.