Rachel pointed out that I left out incendiaries when I wrote up the Apothecary class the other day. At first, it was because I was mostly focusing on drugs, in keeping with the literal meaning of "apothecary". But the class is meant to be more than a pharmacist; non-medicinal compounds are included in the class ability list in order to broaden the class's usefulness.
We've got four basic compounds that might be considered incendiaries:
Accelerants basically counteract an adverse condition. Assume that anyone with appropriate gear can light a torch or build a campfire without a roll under good conditions, but can't if they don't have tinder, the wood is damp, or the substance is hard to ignite (but still combustible.) For each condition, there's a compound that will counteract that condition. The form of the compound is usually liquid, but just use the table from the previous post to create other varieties of accelerant. These all require a spark or small flame of some kind.
Self-Igniting Compounds do not need a spark or flame; they catch fire when they contact a particular substance, are struck by a blow, or after degrading over time. Treat the base form as a paste or salve (requires Level 2,) although other forms are possible.
Fuels are exactly what they sound like; they do not require additional fuel, but do require a spark or flame to ignite, which might be the result of a self-igniting compound mixed in with the fuel. The canonical example is flaming oil. Powder fuels (3rd level) can be used to create flame jets. Incense (4th level) can be used to model flammable gasses. Flaming oil uses the pool of acid as a model.
These three do not cause damage themselves; the fire they cause does the damage. Therefore, the base cost is the same as any other non-poison; the cost of flaming oil should be 60 x 2d6, the theoretical cost of an oil that spreads into a pool like acid, but does no damage. Add the cost of any other compounds mixed in, like self-igniting flaming oil (combined cost: 80 x 2d6.)
Explosives, on the other hand, do cause damage extra damage, so use the poison cost for the appropriate form (powder, incense, or oil.) Again, increase the cost if other compounds are mixed in, such as self-igniting explosive powder (300 + 20 x 2d6.)