
MurphysParadox |

Well, it looks like a dagger of venom is a specific weapon version of a +1 dagger that costs 8302g. I've not seen rules on improving specific weapons, but you could make the argument that the price breakdown is thus:
Dagger = 2g
Masterwork = 300g
+1 bonus = 2000g
Venom = 6000g
So the cost to upgrade it is the standard 8000 - 2000 = 6000g. So a +2 dagger of venom would be 14302g.

Brain in a Jar |

I would treat Venom as a +1 bonus cost. (due to the fact that a Dagger of venom costs the same as a +1 flaming dagger, 8000 gp + masterwork + weapon cost).
Thus a +2 dagger of venom would be a total of a +3 weapon for costs 18,302 gp.
(at least that is how I would work it out)
Actually it works like this. (Also i know they break it down in Ultimate Equipment i just don't have it yet.)
The Dagger of Venom is just a +1 Dagger, it even says as much in it's description.
+1 Dagger = 2,302 Gold
Then it has a Use Activated Poison effect at Caster Level 5th, 1/Day.
Use Activated = Spell Level(3)x Caster Level(5)x 2,000= 30,000
Charges Per Day(1/Day)= 5/Charges Per Day(1)=5
30,000/5= 6,000
So the Poison effect costs 6,000 Gold.