On your list of poisons you have Arsenic costing 120 gp (rounded down from 123, I assume), and note that this price is before rarity modifiers are factored in. Yet, in your example of how to calculate poison prices (at the bottom of the main Poison page), you achieve the figure of 120 gp by including rarity (a .5 modifier). Further, I think you missed a step in your calculations; that is, the increase in cost due to the Type of poison.

Arsenic is a mild nuerotoxin, so 60 gp (60 x 1) is added to the base price (25). The DC is 13 (+30) , primary damage 1 point (+20), secondary damage 4.5 (+45) for a subtotal of 180 gp. It's ingested, so multiply by 1.5, for a before-rarity-cost of 270 gp. This makes the final cost, including a 'common rarity' modifier of .5, 135 gp.

Arsenic is not the only victim of this error, if it is indeed an error.

Black adder venom is another mild nuerotoxin. Base (25), mild nuerotoxin (+60), save DC 11 (+10), primary damage 3.5 (+70), secondary damage 3.5 (+35), for a subtotal of 200. It's injury-based (x1), so the before-rarity-factor cost is 200 gp, not the 185 that you have listed.

Now, had you calulating with a DC of 12 (as it is in the DMG; you've since changed it to 11), and been forgetting the 60 gp due to Type costs (nuerotoxin = +60 gp x strength rating), and been multiplying secondary damage by 20 (as you did in the example, and not by 10, as the table says one should), then indeed the before-rarity-factor cost would be 185 gp.

I think the other poisons may have also been mispriced by the same errors; I have not checked all of them. I just thought I would bring this to your attention. Sorry if my post is hard to understand at any point.