It's probable that Diablo kept making deals with dictators and committing crimes to finance his potions, so while his personal finances are merely GD, he regularly dealt with small countries with greater resources.
Right off the bat, you could add up to 3 special requirements (ingredients) to the potion, at -1CS cost each.
That brings you down to RM cost. To keep it from turning into a massive series of fetch quests, maybe two of them are actually common, but were called by different names in ancient times. Perhaps our current knowledge of these ingredients is flawed, due to a mis-translation of a partial text, so the hero needs some archaeology or language skills to determine what's actually needed. "Great Scott! They used oleander instead of garlic!" Something like that. The effects are probably temporary and limited to a single power such as Ultimate Skill, Total Memory, Linguistics, Hyper-Invention, or Hyper-Intelligence. Lots of ancient rituals involved the solar calendar so maybe you can only brew the stuff by the light of the summer solstice, too.
For the really special ingredient, e.g. alchemical mercury, the hero might need some mystical advice provided by another character or a consultation from a really long-lived contact. Asgardians or other deified races could shed light on the subject, or try to conceal the knowledge.