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.
Magic Find is calculated using the Increased Item Quantity(IIQ) and Increased Item Rarity(IIR) of the player that gets credit for the kill. If a minion gets the kill, then any IIQ/IIR from support gems affecting that minion are applied first.
From my experience this also affects opening chest, so when we go out in a party, we usually have the person with the highest MF open the chests.
I think IIQ is the only stat that will give you more orbs, while IIR increases the rarity (color) of an item.
Edit Thanks to Kexlox for finding this information on IIQ in parties:
When in a party, each player in the party after the first gives a +50% item quantity modifier on drops.
Source: Mechanics Thread
Best Answer
Portal gem is the only unique gem in the game, and, you guessed it, it opens a town portal =)
The upside: Never have to use a TP scroll again! You can save them and trade them for ID scrolls! Fits in any color slot. And you don't really need to "bind a key"... You can just place it on one of the keys when you need it, or put it on your secondary weapons. (that's what I do)
The downside: It has a cast time. At 20% increased quailty, it becomes 40% decreased cast time.
And yes, they are quite rare.
Sources: I have one, wiki
Alas, I died, so no longer have my precious Portal Gem.