I'd also love such a method, but unfortunately it's something that hasn't been provided. The BoVD simply describes crafting poisons as being the same as crafting anything else, except in regards to cost and time.
So
http://www.d20srd.org/srd/skills/craft.htm becomes the fallback resource, and it's vague. We can see that crafting drow poison is about as difficult as alchemist's acid, or a martial weapon, or a bell, which tells us nothing at all.
So here's some
guidelines and houserules that I suggest using, based on having read about 2 dozen poison charts for this guide:
DC 12: The poison should be extremely simple and nearly ineffective. Only 1 poison (Salvo, C. Adv.) is this easy to craft.
DC 15: If the effect is any kind of ability damage/ability damage such as 1d4 Str/1d4 Str and has a save DC under 16, it is probably DC 15 to craft. If the effect is any kind of status effect with a save DC under 17, it will also be DC 15 to craft. DC 15 also covers most of the poisons with a weak initial effect and a moderate secondary effect like 0/1d6 Con.
- Carrion Crawler Brain Juice
- Medium Spider Venom
DC 18: If the poison has a save DC of 17-19 or deals more ability damage than normal like 1d6 str/1d6 str, it's probably DC 18. A weak contact or inhaled poison might also fit into this category if it had a low save DC.
- Gorgon's Hair
- Elemental Rime
DC 20: If the poison has a save DC of 20-25, or deals more ability damage than normal like 1d8 str/1d8 str, then it's probably DC 20. DC 20 also covers the majority of injury poisons with a weak initial effect and a strong secondary effect, such as 0/3d6 Con. If you don't really know what the poison's DC is and it's injury and doesn't have any unique effects, it's probably DC 20. If it has a unique effect such as slow or paralysis and the save is between DC 15-20, then it's probably a DC 20 craft check.
- Lich Dust
- Large Scoprion Venom (this is weak for the craft DC)
- Purple Worm Poison (this is very good for the craft DC)
DC 25: If it's a contact or inhaled poison with a save DC between 16-20, it generally falls into here. Also this is where you start seeing a bunch of DC 20-25 1d6/2d6 injury poisons, or DC 17-20 2d6/2d6 injury poisons. This is also the highest point that I've seen any ingested poisons reach. It gets really random at this point actually, but I would use this as a safe point to mark the DC of most poisons with a unique effect such as confusion or paralysis that have a save DC of over 20.
DC 30: The poison might have a high DC for its ability damage (colossal scorpion venom), or front-load a lot of damage (Dragon Bile's 3d6 Str/0 Str), or do some unusual damage (like front-loading Int and Cha damage).
- Dragon's Bile
- Colossal Centipede Venom
DC 35: The poison does ludicrous amounts of damage (Black Lotus and its 3d6/3d6 Con damage, but at the low DC of 20), or it might have a stupidly high save DC (DC 54 scorpion venom from the BoVD, I'm looking at you - though you were also updated and nerfed to DC 33 1d10/1d10 in Dungeonscape), or it might have a high DC for an inhaled or contact poison (DC 24 save 2d6 Con/2d6 Con's Vilestar, for example, or the DC 25 save inhaled 1d8 Con/1d8 Con Mist of Nourn)
- Black Lotus
- Colossal Scorpion Venom
Now, since we have the craft DC for colossal Scorpion venom from the BoVD, and we have it updated but without the craft DC from Dungeonscape, we can roughly assume that the poison has maintained the same craft DC - so anything better than colossal scorpion venom probably has a higher craft DC.
DC 40: No listed poisons have a DC this high. But we can extrapolate that a save DC 25-30 inhaled or contact poison with a 2d6/2d6 effect would fall into this range, as would a DC 40+ poison that targeted Con or Dex and dealt around 2d8/2d8 or so. Pit Fiend venom, as an injury poison with a high save DC of 27 and a weak effect followed by death, would probably fall into here, as would Greater Svakalor venom.
DC 45: No listed poisons have a DC this high. But we can extrapolate that a save DC 31-35 inhaled or contact poison with a 2d6/2d6 effect would fall into this range, as would a DC 50+ poison that targeted Con or Dex and dealt around 2d8/2d8 or so. I've put Greensickness at this range for this reason, and probably Megapede poison as well.
Rough value of damage when determining craft DC: Death > Dex/Int/Cha > Con/Status Effects > Strength. (DC +5 for death, +2 for Dex/Int/Cha, -1 for Strength damage)
Rough value of type when determining craft DC: Inhaled > Contact > Injury > Ingested. (DC +5 for inhaled/contact, +0 for injury, -1 for ingested)