I have always hated having two spend a small fortune every time you wanted to do something. I hated spending hundreds of gps on NPC weapon trainers. I hated the thousands of gps associated with unlocking a Weapon of Legacy. I hate 5000 gp diamonds. Not because I object to money being spent. It's just so senseless and arbitrary. 100 gp is like an entire year's income to a peasant. Has anyone noticed that in 3.5, you burn a half pound of silver to enchant a vial of holy water?
Yes, we noticed in our group.
Maybe the only way to get away from these arbitrary cost is using the gp cost as a "background value", but instead require the use of special components. (See also Rels post)
Group these components by tier (or level range) and item type and/or skill.
For example, a quartz might be a level 3 component. It can be used to create a level 3 armor or weapon. If you have 5 of them, you can create a level 8 armor or weapon with it.
But you can't use 25 of them to get a level 13 item. The quartz is to weak for such an item. You'd need an opal for that. But that opal can't be broken down to 5 level 8 quartz or used for 5 level 8 items.
Such level ranges of worth and usability means that exchange rates aren't straightforward, since the exchange process is imprecise. If you don't need a level 13 item, the opal is either stronger or weaker then what you need, and you will lose "worth" in the process.
If you also limit their applicatibilty to types of items and rituals, you get a further constraint. Your Level 13 Opal is great for building Rings, but you actually need Mithral for your sword. So, yeah, maybe you are willing in to trade a Level 13 Opal for a Level 11 lump of Mithral Ore.
You go from currency based trade to barter. By making exchange and "change money" harder to come by, you can avoid a few issues.
Unfortunately, 4E allows the use of Residiuum and Gold Pieces. Though: If you remove Gold Pieces from the equation, and only keep Residium with it "20 % back" rule, you get a good explanation why every merchant will sell you 20 %, but nothing more. Finding a buyer for a +3 Greatsword of Flaming might be possible given enough time, but just "melting it down" to Residuum if that fails means he won't make any losses.