There are two official optional rulesets for crafting
Both are optional and require that you work with your DM as to if they're an option and how they'd work. In either place that these rules appear, they are optional variants to the base rules.
Dungeon Master's Guide
The first official optional rules released appear in the Dungeon Master's Guide in chapter 6 as part of the "Downtime Activities" section:
Crafting a Magic Item
Magic items are the DM’s purview, so you decide how they fall into the party’s possession. As an option, you can allow player characters to craft magic items.
Within these rules, there are the following requirements:
- a formula that describes the construction of the item
- the character must also be a spellcaster with spell slots and must be able to cast any spells that the item can produce (and expends the relevant spell cost each day during crafting)
- the character must meet a level minimum determined by the item’s rarity
- a required gold and time cost to craft the item during downtime (consumable cost half as much as other magic items)
\begin{array}{c|c|c}
\text{Rarity}&\text{Cost}&\text{Minimum Level}\\
\hline \text{Common}&\text{100 gp}&\text{3}\\
\text{Uncommon}&\text{500 gp}&\text{3}\\
\text{Rare}&\text{5,000 gp}&\text{6}\\
\text{Very Rare}&\text{50,000 gp}&\text{11}\\
\text{Legendary}&\text{500,000 gp}&\text{17}\\
\end{array}
Xanathar's Guide to Everything
An alternative ruleset is part of the optional variant to downtime as a whole (and one I personally think is more fleshed out) that appears in the supplement Xanathar's Guide to Everything in the "Downtime Revisited" section of chapter 2:
Crafting Magic Items. Creating a magic item requires more than just time, effort, and materials. It is a long-term process that involves one or more adventures to track down rare materials and the lore needed to create the item.
Within these rules, there are the following requirements:
- a formula for a magic item in order to create it
- an exotic material to complete it. This material can range from the skin of a yeti to a vial of water taken from a whirlpool on the Elemental Plane of Water. Finding that material should take place as part of an adventure. (the suggested challenge rating of the creature to face, though not necessarily harvest the material from, appears in the table below)
- whatever tool proficiency is appropriate, as for crafting a nonmagical object, or proficiency in the Arcana skill.
- a required gold and time cost to craft the item during downtime (consumable cost half as much as other magic items). Do note that downtime rules as a whole are revised when using the Xanathar's Guide to Everything rules
\begin{array}{c|c|c|c}
\text{Rarity}&\text{Challenge Rating}&\text{Time}&\text{Cost}\\
\hline \text{Common}&\text{1-3}&\text{1 workweek}&\text{50 gp}\\
\text{Uncommon}&\text{4-8}&\text{2 workweeks}&\text{200 gp}\\
\text{Rare}&\text{9-12}&\text{10 workweeks}&\text{2,000 gp}\\
\text{Very Rare}&\text{13-18}&\text{25 workweeks}&\text{20,000 gp}\\
\text{Legendary}&\text{19+}&\text{50 workweeks}&\text{100,000 gp}\\
\end{array}
This is explained in the Rarity section of Magic Items on page 135 of the DMG:
Common magic items, such as a potion of healing, are the most plentiful. Some legendary items, such as the Apparatus of Kwalish, are unique. [...] Rarity provides a rough measure of an item's power relative to other magic items. Each rarity corresponds to character level, as shown in the Magic Item Rarity table. A character doesn't typically find a rare magic item, for example, until around 5th level. [...] If your campaign allows for trade in magic items, rarity can also help you set prices for them.
Aside from being a rough measure of power, rarity, as somewhat stated above and by its very name, also provides a guideline for frequently characters might find a certain type of magic item.
A legendary magic item is far more powerful and likely to be far more scarce than an common or uncommon magic item. Characters might find multiple Cloaks of Protection (uncommon), for example, throughout their adventures but would probably only ever find one Robe of the Archmagi.
Depending on the type of game your DM runs, obtaining magic items may be a rare occurrence but that still doesn't necessarily mean that all magic items are rare or that there's only ever only one of a magic item in existence.
Best Answer
WFRP, 2nd Edition, p.169:
(All emphasis mine.)
I don't have the other books handy, but I strongly suspect this approach has been rather consistent through the editions. We've always played it this way, at least: a simple, single magic item in WFRP is, imo, about the equivalent of a minor artifact in DnD in terms of value, rarity and importance.