I think most people are missing the key point of what's going on with the poison here. The rules are giving you a game effect. Not a story effect.
The average person isn't going to get their hands on this poison. Hell, the richest king in all the world, despite likely having more than enough money to afford it by the gallons, has no way of actually purchasing it. If he had the means to aquire such a powerful poison, he'd likely be emperor of the world, not merely a king. It would likely take less than a tenth of a drop to kill most people outright. But that's a story element. The game doesn't need rules for that. If your character gets his hands on some and wants to use it to slay a king who's not an epic-level warrior, chances are the DM's gonna say he simply dies if you manage to find a way to actually administer the poison.
The price and effect are based on epic-level play. Sure, it's expensive. You could buy up a gigantic chunk of land for that price and start building your own kingdom. But doesn't that price seem more than reasonable for a poison that's capable of even effecting greater demons and the likes of the Terrasque? Lesser poisons would be broken down instantly, but this one is so toxic it has a chance of doing at least some damage before it's washed away. I mean, we're talking about epic foes with epic bodies. We're talking dragons that could demolish continents, demons capable of ripping the world apart at the seems, divine entities! Is it really so shocking to believe that they'd be able to shrug off even the universe's most vile toxins with relative ease? I mean, we're dealing with creatures capable of fighting toe-to-toe with a Pit Fiend, and the last thing on their mind while doing so would be watching out for the devil's tail.
It seems to be if you'd want something powerful enough to truly cripple an epic-level foe, you'd be looking at a poison operating under relic-like rules. Something that would take months or even years of questing to find or create yourself.
The Drought of Death. A mixture of poisons from every toxic plant and every venomous creature in existence, bound and strengthened by reagants that knowledge of was long since lost. Said to be powerful enough to slay even the most ancient of dragons and strongest of demon lords.
Asmodeus's Tear. The single tear said to have been shed by Asmodeus as he slew his god. A substance so vile it burned a hole in the very fabric of the Astral Sea and was lost to the cosmos. The gods themselves fear even whispered rumors that it may have been found, for they know the tear's corrosive properties could melt their very beings into nothingness.
Pit Toxin? To a greater demon, it's no more than the bite of a mildly poisoness spider to the average human. Sure it's dangerous and painful. But it's not lethal.