You really don't need rules because published rules can't cover every setting. The price for anything will be governed by the setting.
What is the relative wealth of the setting? Wealthier settings would have inflated prices on pretty much everything because even the average commoner has cash to spend. In poorer settings the costs would be down because if anything is priced beyond what the buyer can spend then the seller is not making the sale but is instead stuck with inventory.
What is the magic level of the setting? Let's face it, in a magical setting poisons are often less effective than magic. I'd think that in a high magic setting that poisons would be cheaper than in low magic settings.
How accessible is magical healing? If there is a cleric on every street corner who can Neutralize Poison then the price of poisons goes down... medical aid is just so easy to get for poisons that they lose a lot of their effectiveness. If on the otherhand magical healing is extremely rare then the price goes up. Keep in mind though that the rich and powerful of a setting will still probably have access to magical healing in most cases which only makes poison effective against the lower and middle class.
In the end, if you are the DM I believe poisons should be handled by your own best judgement based upon the setting you are using.
