Old School Rules
According to the DMG, a potion can be made by wizards beginning at 7th level with the assistance of an alchemist, or at 11th level on their own. Having an alchemist assistant will cut production time by 50%. A potion laboratory needs a fireplace plus 200-1,000gp worth of equipment. Monthly upkeep is 10% of the cost. An alchemist earns 300gp per month.
Potions manufacturing time and cost is based on its XP value. The cost to make a potion is 1 GP per XP, time is XP divided by 100, rounded up in days. So a Potion of Healing with an XP value of 250xp costs 250gp to make and takes 3 days. Or a day and a half with an alchemist assistant. That potion sells for 400gp.
So let's do some math. A wizard with an alchemist assistant can make a Potion of Healing every 1.5 days. If they work 6 days a week, that's 16 potions a month. Costs are 4,400gp for materials, upkeep and alchemist pay. The potions sell for 400gp each, so that's 6,400gp or 2,000gp profit. So 24,000gp a year. If you sell all your potions. So why is my wizard going down in dark dungeons? Screw this, I'm staying home!
Reality Check
The original D&D economic rules are based on the idea that you are hauling home thousands, maybe tens of thousands of gold pieces every adventure. So magical stuff costs a lot as a balance on player characters' income. However, I do a less ridiculous game with less gold (and magic) handed out. So I use a "factor of 10" rule. In general, costs for magic materials are reduced by 10 once they go over 1gp. For example, in the books, the Identify spell requires a 100gp pearl as a component. I make it a 10gp pearl. So, looking at the math above, and applying the "factor of 10" rule, Costs are 440gp for materials, upkeep and alchemist pay (which is also cut to 30gp/month). The potions sell for 40gp each, so that's 640gp or 200gp profit. So 2,400gp a year. That's still a pretty tidy income. By comparison, a 30' by 30' wood two floor house will cost you about 360gp.
Poisons on the other hand will probably remain at their high prices because 1, they're illegal, and 2, to keep them out of player hands. Another rule I'm going to apply is that while anyone with the appropriate skill can create herbalism potions or poisons, only spellcasters can create magic potions. I'm also going to modify the above rules for wizards at 7th and 11th levels being able to make potions. That rule predates non-weapon proficiencies. So the new rule is that any wizard with Alchemy proficiency can make potions, two wizards with Alchemy skill or a wizard with Alchemy skill and an alchemist assistant can make them in half the time.
Clerics or druids with Alchemy can make potions that imitate clerical spells, just as wizards can make potions that mimic wizard spells. BUT, clerical potions have alignment restrictions and could be treated as mild poison if used by someone of conflicting alignment.
Gaining skills
Herbalism is a two slot non-weapon proficiency, Alchemy is three. I'm going to add Poison Making (including antidotes) as non-weapon proficiency. Alchemy allows you to make "smoke bombs, sleep gas, magical potions, greek fire, strong glue, various poisons and drugs" according to the article in Dragon magazine #169. That article also says a lab costs 5,000gp, with a -1 bonus for each additional 5,000gp. So, "factor of 10" says 500gp, which is close to the average of the 200-1,000gp cost above. That's what I'm going to use. But just for Herbalism or Poison Making, the cost only 100gp.
I am going to change how you spend slots to get Alchemy, etc. One slot gets you Herbalism, two slots gets you Herbalism and Poison Making, three slots gives you Alchemy which includes the other two skills. This would allow characters to learn Alchemy over time. I will also allow characters to specialize in Herbalism, Poison Making or Potion Making, each extra slot you spend gives you a -2 bonus to that skill.
EDIT EDIT Going to keep Herbalism 2 slots, you can add Poison or Potion Making for 1 slot each. Alchemy is still 3 slots and includes poisons, potions and fantasy chemistry but not Herbalism. You can buy it a slot at a time, the progression is chemistry, poisons, potions.
