The campaign has never put a firm price on fervidite per “unit” …. generally it’s something you can’t buy.
That’s obviously a problem for an Ardakene Warlock from the Pact Magic book. Unless there is a reason why they wouldn’t have access to it in a big city I would assume that they just spend the gold to get a small amount that can be hand waved for weight. Fervidites value is soooo high you can treat its (essentially negligible) mass like diamonds or other fancy gemstones.
(How much does a 10,000gp diamond cost? 10,000gp. How much does it weigh? … Across 5+ editions of D&D I’ve never seen a weight. I just know it easily fits in your pocket)
