[RPG] How to determine the cost of ruby dust

No, buying a cheap pearl for 100 gp doesn't work

The rules say "worth at least 100 gp", not "bought for at least 100 gp". If you buy a cheap pearl from another character for 100 gp, it doesn't become 100 gp worth. And vice versa, if you steal a 100-gp pearl for free, it is still 100 gp worth. This is true for any items, not only for material spell components. The item's cost is determined by a few factors, including the item's quality. So, the item's cost is a (very) rough equivalent of the item's quality. So, why the PH specifies the cost, instead of describing the quality?

Specifying a cost is much easier than describing a quality

Aside from the seller's insights, nothing specific makes an item worth X gp.

"Worth at least X gp" is a short, convenient indication of the item's quality. 5e PH does not describe any specific criteria of "worthing X gp" (hence, being suited for the spell) - it only says the item's guiding price, one number instead of a bunch of words. It's the DM who should decide and say "unfortunately, this pearl is too cheap and small to be used for this spell".

Let's elaborate the pearl example. Instead of "worth at least 100 gp" the PH might describe its minimum weight, radius, material, shape, surface quality, etc. PH had to describe such criteria for every single material component in this way, which contradicts the 5e paradigm. Instead, the PH just says "worth at least 100 gp" - so both DM and players get an idea, what kind of pearl it should be.

Your DM might go the easy way

It really depends on the playstyle, but a DM might decide that market conditions is not a thing in their world. Instead, he/she might take all the prices from the PH and use them as absolute prices, instead of guiding ones. Using static prices model solves many buying/selling questions, but in the end leads to hilarious absurdities as a downside, which might not be welcomed by players.

From a previous edition: AD&D 1e, DMG

Since you are open to previous edition material:

On pages 25-27 of the AD&D 1e DMG, an extended discussion of gem stones almost gets you what you want. I say almost because for size, it uses small, medium, large, etcetera, rather than units of measure we are accustomed to in the real world, like carat or point, or even cm/inch.

The DMG assumed that the DM was generating gems randomly, so:

Score	Modifier	Description	or Size
01-25	10 g.p. each	Ornamental Stones	Very Small
26-50	50 g.p. each	Semi-precious Stones	Small
51-70	100 g.p. each	Fancy Stones	Average
71-90	500 g.p. each	Fancy Stones(Precious)	Large
91-99	1000 g.p. each	Gem Stones	Very Large
00	5000 g.p. each	Gem Stones (Jewels)	Huge

Average Value of a gem depends upon its type, quality and weight. A huge semiprecious stone - carnelian, for example - is worth as much as an average gem stone, quality being equal. Size may vary from stone to stone, a 50 g.p. ornamental stone being of above average size, while a 50 g.p. gem stone would most likely be very small. (DMG, AD&D 1e, p. 25) Usually (under random generation) the stone/gem kind was generated, and then the value /size change checked for.

It then classified which common gemstones fell into the above categories. Two examples.
Example 1 (excerpted):

ORNAMENTAL STONES, Base Value 10 9.p.:

1. Azurite*, 2. Bonded Agate 3. Blue Quartz 4. Eye Agote 5. Hematite' 6. lapis Lazuli*: 7. Malachite* 8. Moss Agate 9. Obsidian* 10. Rhodochrosite* 11. Tiger Eye 12. Turquoise*: light blue-green

Example 2 (excerpted):

FANCY STONES, Base Value 100 to 500 g.p.:

1. Amber 2. Alexandrite (100) 3. Amethyst (100) 4. Aquamarine (500) 5. Chrysoberyl: (100) 6. Coral (100) 7. Garnet (100) (500(violet)) 8. Jade: (100) 9. Jet: (100) 10. Pearl(100) (500) 11. Peridot: (500) 12. Spinel(100) (500) 13. Topaz (500) 14. Tourmaline (100)

For the whole treatment, with tables and more detail, you need the AD&D 1e DMG. It can be purchased online, or you can seek out a nearby grognard and borrow theirs. I can't recreate the whole set of tables without running afoul of fair use rules that SE sites must comply with.

The other neat feature of the AD&D 1e DMG breakdown of gems is the magical properties of gems that are suggested, such as (to show a few examples):

Agate: Restful and safe sleep
Carbuncle: powers of dragon sight
Coral: cures madness, calms weather
Ruby: Gives good luck
Topaz: wards off evil spirits
(1e DMG p. 26-27)

I found those suggestions to be at least as helpful as the value/color/size from the tables in the DMG, when describing some of the magical items that the players found.

Follow up for AD&D 2e: from the DMG, I find that they used the same table, roughly, but provided even less detail. Tables 85 and 86 in the DMG.

Follow up for Dragon Magazine #8 (July 1977): Rob Kuntz created an extended series of roll up tables on pages 22 and 23 of Dragon Magazine issue 8 that went into far more detail than the AD&D 1e DMG ever did. Again, I can't reproduce that without running afoul of fair use.