CraftTweaker doesn't currently support the ability to modify costs (or at least, I don't recall implementing it).

Modifier costs, however, are basically a list of cost instances. The likelihood of there being more than 2 costs is extremely low (but possible), while most modifiers will only have 1 cost. However, it should still be possible to modify these costs via CraftTweaker.

However, like #424 I'd rather not allow people to arbitrarily decide which herb is used instead, as it makes the entire modifier system that much more complicated.