By the book, crafting a magic item (DMG 128-9), you need a formula, a spellcaster capable of casting any spells the item can produce, and a minimum level per item rarity.
The creation cost table is pretty much the same as the salable price (common 100, uncommon 500, rare 5000, very rare 50,000, legendary 500,000), and since progress is tracked in 1-day-is-25-gp increments, that means a common item is 4 days, an uncommon one 20 days, a rare one 200 days, a very rare one 2,000 days, and a legendary one a back-breaking 20,000 days. Luckily, multiple qualifying casters each produce 1 day’s worth of labor per day devoted, so with 100 qualifying casters you would reduce the time necessary to 1% of the initial value (or whatever).
The rules require you cast the spells in the item each day you craft the item. Given my wonky long rest rules, that would be super punitive; I’m happy to waive that.
Okay, all well and good. Let’s see what happens next:
Per the book, a formula creates items one step more common.
I think I’m going to waive that, especially since so many of the costs are so high: instead, a formula crafts items of the same rarity (sidebar on DMG 141, cost double of an equivalent item). Probably a few craft skills substitute for formulae; I’m happy to let wizards scribe scrolls and brew potions if they have brewers tools proficiency or alchemist’s proficiency or calligraphy experience or astronomers tools or whatever, at least for the commons and uncommons.
Any item which is rare or rarer will require 1d6 ingredients from each tier (one use items only take 1d3 ingredients); common reagents can almost always simply be purchased and so are ignored for these purposes.
Uncommon ingredients (approx 50gp each):
Bloodstones, citrines, moonstones, quartzes.
A body part from a CR 5-10 creature (based on individual treasure hauls’ expected value; usually yields 1d6-3 such parts).
As a funny corollary, this means well-collected troll’s blood is worth a healing potion or two.
Treatment at a location within 10 miles (dipped in water from a flowing river).
Rare ingredients (approx 500gp each):
Black pearls, peridots, topazes
A body part from a CR 11-16 creature (based on individual treasure hauls’ expected value; usually yields 1d6-3 such parts)
As a side-note, that means a body part from a CR 11-16 creature is an uncommon item (more or less), so feel free to impart magical properties to the liver of an efreeti or the blood of an archmage; the fangs of a vampire, the armor off an iron golem, the eyes of a beholder, the innards of a mummy lord).
A pound of pure platinum, mithril or adamantium.
An uncommon magical item.
Treatment at a location within 100 miles (dipped in water from a blessed spring).
Very Rare ingredients (approx 5000gp each):
Black sapphires, diamonds, rubies
A body part from a CR 17+ creature (based on individual treasure hauls’ expected value; usually yields 1d6-3 such parts)
A rare magical item.
Treatment at a location within 1000 miles (dipped in lava).
Legendary ingredients (approx 50,000gp each):
A very rare magical item.
Treatment at a location within 10,000 miles (anointed with the Blood of the Redeemer at the First Fane).
My party slew a pair of invisible stalkers (super sad they don’t have a by-default garrotte attack, by the by). They kept the bodies, skinning them and figuring out what to do with the meat. They’re 9th level (so the best they can do are rare items). The bodies don’t even count as a rare ingredient unfortunately (though, sure, they’re 50gp a pop for trade-in value towards an appropriate item).
Looking at the rare items for anything even remotely relevant, that means: a putative Wand of Invisibility, a spell scroll (4th level, improved invis), oil of etherealness (… I guess), a gem of seeing, a cloak of the bat, a cloak of displacement, or a cape of the mountebank.
At uncommons: a spell scroll (2nd level, invisibility), ring of mind shielding (… I guess), a lantern of revealing (… I guess), gloves of thievery (… I guess), dust of disappearance, a cloak of elvenkind.
Let’s say Henderson the mathemagician decides the ring of mind shielding is where it’s at.
That’s an uncommon item: The only requisite is 500 gp on the barrel (400, once he finishes skinning the stalkers) and a formula. And the formula can be purchased for cash-money in the city they’re in, If Only They Could Find A Way To The Wizard Tower (a somewhat recurring theme, though they found a way through a dungeon that might do it!). So 1000 gp and a brief stint in a dungeon startup cost, then 500 gp (less 100 gp in corpse parts), and 20 days per ring.