It should work like this:
* A potion with a positive and negative effect (100 gold worth of fortify magicka + 70 gold worth of Reduice health) should be worth the DIFFERENCE of the values of its two effects (30 gold)
* A potion with non-synergizing effects (fortify one-handed 200 and fortify barter 100) should be worth the GREATER of its two effects. In other words, 200.
* Only a potion with SYNERGIZING effects (300 Fortify Light armor + 150 One-handed) should be worth the sum of its effects, or 450.