If you're playing the potion market to any real degree, I guarantee you that you're not going to be farming your own herbs.

It's a question of time. The time it takes to farm up 2 Whiptail and 1 Cinderbloom might be somewhere around 1 minute on average. At Proudmoore US prices (see chart), that's 3.73g worth of herbs per minute. In that same minute, you can create a full stack of Mythical Mana Potions -- 20 in total. These sell for 10.77g each (see chart), or 215.4g per stack.

If you had farmed the herbs yourself, it will have taken 21 minutes to create the stack -- 20 minutes farming plus 1 minute crafting. You'd make the full 215.4g. This way, you'd have made 9.79 gold/minute.

Now, consider if you buy the herbs on the AH. You don't get as much money from selling the potions -- that 215.4 gold needs to be offset by the 74.6 gold you'll pay for the herbs, so your profit here is only 140.8g. You might spend a whole minute (being very generous) buying the herbs on the AH and grabbing them from the mail, so it'd take you 2 minutes altogether to make this stack. That works out to 70.4 gold/minute.

Even accounting for a handful of Volatile Life here and there, crafting with bought mats will almost always blow farming for your own mats out of the water in terms of how much money you'll make.