I think you could handle this topic like the following:
1) You have a hen that lays eggs. The eggs do not come from nothing they come from the hen. And they have a certain value. Now you can add other ingredients and use profession cook or craft cooking to turn the egg into something more valuable: A meal.
2) You have a venomous animal that gives off venom. The venom does not come from nothing it comes from the venomous creature. It has a certain value. Now you can add other ingredients and use craft poison or craft alchemy to turn the venom into something more valuable: A poison.
How much the egg or the venom is worth is up to the GM as I can't find either on one of the tables. I guess an egg will not be worth more than a chicken is and a live chicken costs 2cp. How much a dose of venom is worth I have no idea. But a viper costs 5gp and a house centipede costs 1cp.
Sadly there is no viper poison and the centipede poison is from the small one, not the tiny one which is purchasable and usable as a familiar.
On the other hand a dose of wolfsbane (medicinal herb) costs 5sp while a dose of wolfsbane poison costs 500gp.