Antivenom is a very potent substance very much worth carrying around if you think that a possibility of encountering a certain monster is real and can get your hands on a vial or two. For example, I’d like my character to have Antivenom against Phase Spiders.
Creating this item is a complex process that requires a dose of target venom. How much can such a thing cost? I think I remember seeing a formula based on monster CR, but I didn’t happen to find it.