The costs in materials to upgrade weapons is completely fucked, and the same goes for crafting weapons and armor, the devs have for some reason given each creature drops 3 different items that do the same thing, Mermaids drop skulls, ail and tail; Wolves skull claw and flesh; Bears, skulls, paws and hearts.
And completely randomly the weapons or armor require one material or another to be upgraded or crafted.
It is a system that makes no sense, because of this system it is always better to use a high level weapon that you find around rather than upgrade the weapon you have.
I recommend you read these two topics, if you are interested: