I just went through (I believe) every possible "pure" potion/poison you can make. By "pure" I mean one effect potions. The purpose of this exercise was to find out which single effects were most valuable. Based on this knowledge, alchemists can create multi-effect potions that combine the most valuable single effects to make a trully expensive potion.

Note that this list was made with a character that has the benefactor perk, but not the poisoner. These values will differ for people that have the poisoner perk and not the benefactor, or have both the poisoner perk and the benefactor perk.

Fortify Carry Weight
Invisibility
Regenerate Health/Stamina/Magicka
Damage Magicka Regen
Slow
Paralysis

A potion that combines some combination of these schools will be one of the most valuable potions available. So for instance, Luna Moth + Vampire Dust makes a very expensive 2 effect potion because it has both Invisibility and Regenerate Health. Combine that with a third ingredient/effect and you will have an enormously expensive potion.

The most expensive potion I have found so far is Giant Toe + Scaly Pholia + Wheat. This is for someone who has the benefactor perk but not the poisoner. If you have the reverse set up, you mileage will differ. Nevertheless, any potions that include fortify carry weight, whether you use Giant Toe or not, should make a very valuable potion.