Ehh... Can't they hear the request first before making a decision? Although I don't think the fairy's gonna tell since they would've already.
...Wouldn't they have got a massive discount by trading the apricot grass they have for the finished potion? IF it's really that hard to find an ingredient.. >.<
Fairy is more of selling their services (using the needed Alchemy skill to turn Apricot Grass to potion) rather than the materials.
While the ingredient is rare (fairy seems to know where to search though and is strong enough to gather on their own), the fairy already said the ingredient Apricot Grass itself
doesn't sell as a raw material since the curse-lifting potion is the much-needed item in the end, which needs a skilled alchemist to produce (fairy can seemingly turn it into potion).