Genuine costus is expensive and hard to get. Costus is endangered and under severe restrictions. It is illegal to export from its native countries, so now it is one of the most illegally trafficked plants. Most "costus root" on the market is fake.

In short, it is better to use another aromatic herb root. Since costus is a thistle and the root is a rhizome, see how she responds to similar plants (ginger, tumeric, galangal, etc).