Modern magic, and the Fingerling

I was just reading this excellent short story from Strange Horizons, "The Duke of Vertumn's Fingerling", and got an idea. The story has a magic spell that is described as requiring a ring of "precious salt." Referring to the salt that way made me realize that salt was of course much more precious in the past than it is today, and gave me an idea for fictional magic.

The story, which is by Elizabeth Caroll, came out just a month ago. It's a little under 6k words long. I liked the hints of colorful magic it contains, and the revelations of its ending. I liked most of all that the author puts us in the head of a creature of magic, a homunculus, and makes her unique. The story is well worth your time.

So here's the idea it sparked. Suppose you have a spellcaster who is researching spells and comes upon a recipe for an old spell. The spell might call for salt, but since the value of salt has changed, its value in the spell might now be negligible. One might need to be astute enough to substitute a modern item that is more precious, or something like gold dust that remains precious. A plot could turn around this, or a character could be distinguished by his ability to make these subsitutions, where others say that the magic fails.