This has actually been a problem since the game first came out in early access. The price of the weapon tends to be tied to the tier of the weapon. The tier of the weapon from what I can tell is tied to the damage the weapon does. The star falcion bladed 2 handed weapon you pictured is one of the most damaging weapons in the game, so it's tier 6 and lotsa money/xp/part unlocks. The harpoon head and any handle for a javelin is also very popular to level smithing and unlock parts because it's exceedingly cheap to make/Tier 6.....you get the picture. They will eventually get into smithing I would imagine. Not sure what they can do with the harpoon head. Has to be easy to make because the sea raiders use it, but have to wonder why sea raiders dont just sell overpriced javelins lol
Another thing about javelins that makes me think the Devs haven't had a chance to look into smithing yet is there is a handle that costs 0 resources to make. Think it's the ash handle. It's the one in the middle. So, if you have the harpoon head discovered and that handle you can make one with 1 iron, 1 charcoal and it sells (depending on your actual smithing level) for the max 100k+. Reason I think this illustrates the devs haven't had time to dig into smithing yet is because it should be relatively easy to make the handle cost 1 wood.