One of my players asked about in-play use of the Getting Stuff Cheap rules (Dungeon Fantasy 2: Dungeons, p. 3-4) - specifically crafting. By extension, this should cover brewing, as well.
Those rules are meant for starting gear. They can get you a very steep discount on an easy skill roll in play. However, it's reasonable to allow PCs some access to them in game. Let's rule on how to do that:
Crafting works as listed. Appropriate Armoury specializations can be used to craft weapons and armor without prefixes. Crafting a weapon requires access to the appropriate tools and takes one month. Additionally, critical failure doubles the cost, and an 18 destroys the raw materials entirely.
Ammunition can be crafted in batches - 5 for arrows, bolts, or sling bullets - and takes a day per 5.
Brewing works as listed. However, one dose of a given elixir, concoction, etc. can be made per week; each additional dose attempted at the same time is a cumulative -1 to the roll. Additionally, critical failure doubles the cost, and an 18 destroys the raw materials entirely.
Characters who want a bonus for extra time or a penalty for getting it done more quickly can use the usual rules for time (Campaigns, p. 346).
With either of these, they take up the time you'd spend shopping for items that require an availability roll, gathering rumors, etc. So you couldn't, say, learn multiple spells in a week and brew up some potions. That should effectively cap this being a 98.1% chance of a discount for potions, for example.
As usual, I don't allow Luck on rolls that reflect a serious of cumulative actions.