I have some house rules for brewing poisons. They fit the ones in the DMG pretty good.
You have to be pretty clever (and pretty high level) to make a DC 20 poison. And instant unconsciousness is pretty expensive (at least using the formula I derived from Carrion Crawler Juice).
I get 1270 gp per dose of injury poison, according to my rules. Which are as follows:
Poison
The maximum save DC that a poison can have is based on the brewer’s character level and intelligence modifier. The exact formula is
10 + ½ level + Intelligence Modifier
The DC to brew the potion is equal to its save DC + 9.
The base cost of a poison is determined as follows:
[(save DC –9) x 10 + (average primary damage) x 20 + (average secondary damage x 10)]
One point of permanent ability damage costs as much as 1d6 of temporary ability damage. Unconsciousness (as a secondary effect only) counts as 5 points (treat as paralysis if it's a primary effect). The base cost is increased by certain modifiers, and a final “decanting” cost of 25 gp is added to determine the final price.
Injury poisons do not have a special modifier. Add +700% for inhaled poison. Ingested poison has a –40% modifier if it affects one mental stat or induces unconsciousness, -10% if it modifies multiple mental stats or includes physical effects. The cost of contact poisons is +300%.
Example prices (DMG price)
Injury poisons
Small centipede poison 90 gp (90 gp)
Greenblood oil 100 gp (100 gp)
Medium-size spider venom 160 gp (150 gp)
Bloodroot 100 gp (100 gp)
Large scorpion venom 220 gp (200 gp)
Blue whinnis 145 gp (120 gp)
Giant wasp poison 220 gp (210 gp)
Shadow essence 245 gp (250 gp)
Black adder venom 90 gp (120 gp)
Contact poisons
Dragon Bile 1545 gp (1500 gp)
Malyss root paste 585 gp (500 gp)
Nitharit 605 gp (650 gp)
Terinav root 865 gp (750 gp)
Ingested poisons
Oil of taggit 91 gp (90 gp)
Id moss 127 gp (125 gp)
Striped toadstool 146.5 gp (180 gp)
Arsenic 119.5 gp (120 gp)
Lich dust 254.5 gp (250 gp)
Dark reaver powder 295 gp (300 gp)
Inhaled poisons
Ungol dust 1225 gp (1000 gp)
Burnt othur fumes 2145 gp (2100 gp)
Insanity mist 1465 gp (1500 gp)
Paralyis poison is hard to judge; (Save DC –9) x (Save DC - 9) x 10 seems to work for Carrion crawler brain juice (160 gp +25 gp = 185 gp vs. 200 gp).
Some poisons are off the scale; they are Con poisons with high DCs and hefty con penalties. There is no way that Wyvern poison should be more expensive that black lotus extract.
Deathblade (Injury 20, 1d6 Con / 2d6 Con) 250 gp (1800 gp)
Wyvern poison (Injury 17, 2d6 Con / 2d6 Con) 290 gp (3000 gp)
Black lotus extract (Contact 20 3d6 Con / 3d6 Con) 1700 gp (2500 gp)
I would suggest that Deathblade and Wyvern Poison both be considered contact poisons worth 1025 gp and 1185 gp per dose respectively.
Conversely, Sassone leaf residue should cost 445 gp based on the DC and ability damage alone- the DMG price is only 300 gp. I would suggest it being labelled an injury poison, with the base price given by
(save DC – 9) x 10 + (primary average hp damage squared) + (secondary average ability damage) x 10
Then the price would be 80 + 169 + 35 + 25 (decanting) = 309 gp (300 gp)
