[RPG] Is it possible to reduce the cost of brewing potions

Trophies

Ultimate Wilderness adds the (optional) rule to collect Trophies from defeated enemies, which are parts of their bodies that have some value, either as decoration or for their mystical properties. The latter actually discounting the creation of magic items that have affinity with the trophy:

Certain creatures provide trophy components that, once processed into actual trophies, are exceptionally useful for the construction of alchemical or magic items. When used as raw materials for the crafting of alchemical or magic items, these trophies are worth more than their normal values for the purposes of calculating the total gp needed to craft the item. (...)

The magical uses for a trophy in the construction of alchemical or magic items are listed here. When a trophy is incorporated into raw materials, its gp value is considered to be 20% greater than normal.

For instance, devil's hearts can be used to create magic items that require spells with the [evil] descriptor, giving you a discount. The heart of an Osyluth (CR 9, 10 HD) is worth 850 gp in the market, and for magic items that require an [evil] spell, it would be worth 1020 gp instead when used as magical supply to create magic items.

The Monster Hunter Handbook introduces another way to harvest enemies, using the Harvest Parts feat.

You can attempt a Craft or Heal check, as though making a trophy, to gain usable resources from a creature that has been dead for less than an hour. Only creatures with a CR of 1 or higher yield usable parts. The value of the parts you harvest is equal to the creature’s CR squared × 10 gp (increases to CR derived from class levels or templates do not contribute to this value). This value can be used only as raw materials for crafting alchemical, masterwork, mundane, or magic items. Items crafted using creature parts must be made of a suitable material—typically bone or hide, with metal only in extraordinary cases. No more than a quarter of a crafted item’s cost can be supplied with harvested parts. Harvested parts remain usable for 2 days before they rot (unless used to craft objects or somehow preserved). Creature parts that are harvested in this manner can’t be bought or sold in most settlements.

While those are slightly less valuable, you are not restricted by the creature's affinities. You are also limited to 25% of the item's cost using those supplies.

Downtime

Using the (optional) rules for Downtime, you may obtain capital which can be used to craft magic items. Those are abstract magical supplies that can be used to all sort of things, not only crafting, and can be flavored as "gathered herbs to create potions".

Magic: Magic represents magical power at your disposal. Some activities, such as healing sick peasants in the slums or constructing a magical library, specifically require you to spend Magic.

Each Magic capital is worth 100 gp, and to Earn them it costs 50 gp (half the Purchase price). And they may be utilized in crafting magic items just as if they were magical supplies.

The standard rules detail how to craft magic items. As magic item crafting and the downtime rules both use days as time increments for all but the cheapest potions and scrolls, you can spend days in the downtime system to craft magic items, with each downtime day counting as 8 hours of crafting time. You may spend Magic toward the crafting cost.

You may also gather herbs as Goods (20 gp to buy, 10 gp to earn), and later convert them to Magic:

You can trade 3 points of Goods, Labor, Influence, or Magic for 1 point of Goods, Labor, or Influence. Under certain circumstances, the GM may allow you to trade these resources at a 2-for-1 rate rather than the normal 3-for-1. You can trade 5 points of Goods, Labor, or Influence for 1 point of Magic.

Keep in mind that this could potentially reduce the cost of creating magic items in half, as it costs half the capital value to earn them, with the only downside being that it could take much longer than simply purchasing the capital or magical supplies. Talk with your GM if it's okay to do that, as some GM's aren't keen on methods that increase the party's wealth.