Does anyone know which script is responsible for determining the price of a potion? I haven't had any luck finding this so far. I'm interested in making a mod that will alter the value of potions created through alchemy such that a potion with both positive and negative effects is less valuable. Thanks!

This is kind of a guess, but I've noticed that if you double-click on a potion in the CK, you will see a table listing the various effects of the potion features along with the associated cost. Below this table is a field labled "Potion Value" along with a check-mark box for "Auto-Calculate". You could just uncheck the "Auto-calc" box and type in any value you want for a given potion.

Then again, perhaps you already saw this. I'm not even sure if this is the actual monetary value for a potion... the table might be referring to the magic costs for spells, rather than an actual value.

The potion value of a custom potion is determined by the magic effect base cost and the potion strength. And nothing else. There is no way to change the way it is handled as that part of the game is hard-coded. Additional effects on a potion will be added to the total price, whether it's a negative or a positive effect doesn't matter.