One point in "extract potion" should take care of your potion needs for some time; if that's not enough, another in "extract" and two in "combine" will let you make medium restoration potions, which fully restore health and mana. If not, you might think about your character design, and look into getting more defense. Speed, agility and survival shouldn't be neglected.
I sell a lot of potions, but haven't had to buy any yet!
