This is how it works in Warhammer:
"To use ruby sulphur extract, it must either be mixed in with food or drink, or inhaled, such as by rubbing the powder on a handkerchief or scarf. A poisoned victim must succeed a Challenging (–10%) Toughness Test or die in a number of hours equal to his TB."
TB is Toughness Bonus. It's colorless, odorless and tasteless and can be mixed with anything, and it's very rare, so that inflates its value. Manticore Spoor is fairly obvious in its effects, whereas Arsenic allows you to delay the death so you can attempt to be very stealthy with it. Arsenic caused a series of unexplained deaths with absolutely no evidence found for them until the Estalian government found an assassin with the poison in their possession, allowing people to find out what the new poison was.
Obviously, the price might be inflated by the mystique. Also, I was a bit inaccurate. It's only found in one place in the Old World. Cathayan caravans sometimes bring them over from the Far East, but that's obviously super expensive too.
To provide an example of another super deadly poison that's very close in price range (800 gold crowns), I present Heartkill:
"Heartkill (Hard): Certain potent venoms become even deadlier when mixed. Such is the case of the amphisbaena (the two-headed serpent) and jabberwock venom. When blended, these toxins form a colourless, odourless liquid, virtually undetectable when placed in food or drink. The imbiber must succeed on a Challenging (–10%) Toughness Test, or die in 2d10 rounds when the poison reaches the heart and kills the drinker." Page 72 Old World Armory 2E
Heartkill being so much faster is probably a disadvantage for subtle assassins.