If I understand it properly, I think there's a problem in the question itself. As you said:
The merchants give you Scroll Fragments for white items, so we can sort of divine that that is the base item value. But it doesn't really flow well from there [...]
It doesn't add up, indeed, and you gave the exact reason in your own question:
- Path of Exile has done away with a single unified currency entirely. Exact
- [...] there's no way to tell what anything is worth, or which items are worth more as part of the local economy. More or less correct
Actually, while there isn't any curve to express the value of the items in terms of value, we probably could figure one out based on the difficulty to obtain the item from the vendors.
- You can get a
Scroll Fragment with pretty much any crap item (which are easy to find).
- You can make a
Scroll of Wisdom with 5 Scroll Fragments (which are easy to find, but you need 5 of those).
- You can get a
Portal Scroll with Scroll Fragments (but you need 15 of those). It's getting harder to obtain, but still feasible.
- You can get an
Armorer's Scrap with an piece of armour with a 20% quality, or with several pieces of armor with a total of 40% quality: feasible, but you'll need patience.
- You can get a
Chromatic Orb with any item that has 3 sockets of different colors and linked together: you'll have to be very patient to find one of these.
- You can get an
Orb of Chance with 2 rare items that share the exact same name: it's getting super hard.
- Etc.
With that in mind, the whole currency thing becomes a matter of recipes, and the "value" of each currency item is determined by its difficulty to be "cooked" (let's put it this way) and, therefore, its rarity.
The vendor recipe system allows the player to sell items to any town vendor in exchange for a multitude of currency items and equipment. Each recipe requires semi-specific items or combinations of items be put into the sell window at the same time, and the outcome will change based on any recipes that have been matched.
Source and list of known recipes: http://pathofexile.gamepedia.com/Vendor_recipe_system
The only remaining uncertainty in this system is the drop rate of each currency item (because, of course, it's also possible to obtain them by killing mobs). But I couldn't find any information about it.
The dev diary given by Gnoupi is also an excellent reading: Dev Diary: Rethinking Gold as a Currency.
From the mechanics thread:
Increased Item Rarity
Increased Item Rarity % modifiers increase the chances of an item being magic, rare, or unique. For example with a total of +100% increased item rarity, you'd get twice as many magic items, twice as many rares and twice as many uniques from normal enemies.
This modifier has no effect on the number or type of currency items, scrolls, or gems that drop.
When in a party, only the modifier from the player who lands the killing blow on an enemy is counted.
If one of your minions gets a kill, the minion's IIR is added to yours and the total is used.
Magic, rare, and unique monsters have an Increased item rarity modifier for drops.
Best Answer
Update
According to this post by Rhys, most uniques can actually spawn on any item of the correct base type, regardless of item level:
As far as unique Onyx Amulets go, the minimum item level is still 20, as that's the minimum item level for Onyx Amulets in general.
Previous answer
Just as for normal items, the minimum item level of a unique item is the same as the minimum level to equip it. The unique Onyx Amulets all require level 20, just like a normal one, so any Onyx Amulet you find could potentially be chanced into any of the unique ones (bar any exceptions set by GGG to prevent new copies from being obtainable).
It's kind of rare for a unique item to have a different required level from its normal counterpart, but exceptions do exist (e.g. the new Voll's Devotion Agate Amulet is level 32 while a normal Agate's is 16). If there's a particular unique you're interested in, look it up on the Path of Exile wiki.