I have two fairly similar helmets, but one is seemingly worth about eight times as much as the other one. I can't see a reason for why it's so valuable? It's the Bull's Helm of Matter.

Items: http://i.imgur.com/g35JoPp.png

The difference is that the most valuable one has +2 to constitution, while the other one has poison resistance and Charm capability. +2 constitution can't be the reason it's so expensive? Unlike the cheaper one it doesn't even have Charm.

Walker the apothecary is ready to pay a lot more than others I've talked to, but they too are willing to pay 15K+ for it.

I'm not sure how prices are established in the game, but I assume something must have gone wrong. Seems to me the other one should be more expensive.

Latest GoG version.