I'd like to design a 4E encounter with some monsters using poisoned weapons, but I am a little unsure of how to factor this into the XP budget. The DMG (pg 50) gives the GP cost and level for various poisons - does this mean I should treat a 5th level poison as the XP equivalent of a 5th level monster (200xp)? That seems like a pretty steep price, but I suppose poisons can provide significant tactical advantages... I think the same question goes for magic weapons. I mean, it seems far more dramatic that the party pry the Gauntlets of Gruumsh from the cold dead hands of their foes, rather than find them moldering away in some goblin's closet. Or is there a better way to calculate this?
The DMG anticipated this, and has a mechanic to compensate
It's a little complicated, but DMG 187 introduces a mechanic called the "Magic Threshold." It is designed to let you give an NPC a powerful magic item without imbalancing the PC, by reducing the bonuses the item grants that NPC. If you use this system, it replaces any need for XP adjustment based on NPCs' use of powerful items.
Poisons, being consumable items, can be balanced easily by applying the Magic Threshold penalty. Poisons with powerful non-damage effects may require some additional adjudication depending on how well they support the other mechanics in your encounter (I would use the poison's effect to mimic or replace an existing rider on the NPC's attacks, so it all balances out).