My first piece of rich-media, and I’m wasting it on this.
Continuing the theme of reagents, here are my further thoughts.
I don’t see how to avoid giving these reagents gp equivalents. Whatever spell they’re a gp equivalent for, we’ll call their price. There will be a few excursions from this; items which are over- or under- priced as a result of the spells they’re tied to or something, but that’s the base idea.
So given that I’ve decided these are sort of alchemical items, what sorts of items are we looking at? Let’s measure the intended price of the reagent against other items, gems a base.
Common (10gp) reagents: Obviously, these are the 10gp gemstones. But they’re also a broad swath of the equipment chapter of the PHB: 50′ of silk rope, a shortsword, a bottle of wine, 2/3 of a pound of saffron, even just an ounce of ink. These are things that can be grown in gardens and harvested from animals. Like, I don’t know if I’d permit eye of toad or wing of bat, but I’d definitely do tiger’s chaudron or a murderer’s hand or a virgin’s wedding-nighttears.
Uncommon (100gp) reagents: quelle suprise, the 100 gp gems. This is a more restricted segment of the PHB; vials of poison, a magnifying glass, 2 flasks of alchemist’s fire, 2 lbs of gold, or a riding horse with tackle. This might include monster parts from the CR 3-5 range, or byproducts (dragon’s tears etc) from above that range. It might also include wine made by a duke’s hand, flowers or fruit or herbs picked at the peak of freshness (and only good for a limited time), a martyr’s bodyparts or the prayers of a few monks for a year, or other time-limited but otherwise attainable ingredients. I think mundane abstract quantities show up here, too: bottled ennui, parched discomfort, powdered tears, petrified fear.
Rare (1000gp) reagents: the 1000gp gems, obviously. PHB comparison items start topping out here; 2 pound of platinum, so rare minerals are still in play, but otherwise things like spyglasses, small ships, or platemail. If they’re not incredibly rare substances, and we’re pushing the limits of herbs here (perhaps a bloom which grows once a decade or something?), they’re highly refined cultural products. Maybe uncommon potions can be infused with 3 pounds of gold and distilled into something useable here; a lord’s child, or the prayers of 30 monks for a year. Intricately inlaid tangles of precious wire, extremely purified chemical reagents, positive abstract quantities start to show up (a maiden’s blush in a kerchief, a perfect summer dawn in a bottle, the age of three winters).
Very Rare (10,000gp) reagents: no mere gem table can contain these; they’re named and unique. In terms of finished goods they’re mid-sized ships and towers, small permanent structures that take many hands to render. That makes it difficult to name a similar hand-held object; not too many devices require the same sophisticated cultural apparatus. Ludicrous jewels and ludicrous-jewel-encrusted-objects count, though. We’re looking at once-in-a-century picked-by-a-virgin-with-a-silver-sickle flowers and fruit, or perfect anatomical replicas worked in gemstone, or the keygrain of a desert, the root of a mountain. You might have a bottle of wine filtered through the ashes of its vinyard, or a heartroot of a century-tree.