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.
Best Answer
You can initially have up to three charges of each kind (power, frenzy, and endurance), which will last for 10 seconds - but as you've noticed, you can refresh this duration indefinitely.
This can be increased by an additional 3 charges and roughly 3.5-4 seconds (I'm not sure how those bonuses stack, but the difference is small enough) via passive skills.
Helping Alira in the Merciless Act 2 bandit quest provides another +1 max power charges.
Two-handed weapons can be corrupted to provide +1 max power charges.
The unique staff Hegemony's Era can provide +1 maximum power charges, or +2 if corrupted with the right mod.
The unique wands Void Battery and Storm Prison also provide + 1 maximum power charges, allowing for 2 more max charges if duel wielded.
The unique amulet The Aylardex can provide +1 maximum power charges.
This leaves us with up to 10 max power charges, depending on gear chosen.