[@Vertigo] Not a dumb question at all, as [@Ellri] said 5e is a bit light on info like this.

Pay for the cost prior to delving into the dungeon and make a note of the total cost next to your alchemy supplies, then deduct the amount when you do the crafting (bearing in mind rolling well might decrease the cost and vice bersa). You may also find things like, (x GP amount) of alchemical supplies as loot during the game if we're using the lite rules.

If we use the more in depth rules, let me know and I can tell you specific ingredients you have to hand/find and you can make more customisable potions. But then we wont use flat gold values, so you'll have more specific options and control over your concoctions but it'll be less straightforward. (e.g. I combine the Wild Sageroot with the dried Ephedra that I've preserved in my Ingredient Pouch)