It doesn't have to be metal; as I said, converting it into Biostructure at the end is a pretty damn good idea.
So is kaleidomantics.
The whole idea was "what is the cheapest way to secure a doorway that can be built?" Because not everyone will have the cash to grab a Magisterial level Grammarist.
Also, let's talk currency. I know, I know, our friends the Grammarists can't create money-grade gold/silver, but why risk it? After all, you can doctor the gold/silver, and, hell, grammaric gold and silver are probably more valuable pound-by-pound than their non-grammaric counterparts (they're easier to get your hands on, it probably has been modified in other ways...)
Also, you can "Fool's Gold" it up, using Imachination to make fucking pebbles into gold bricks.
So, I'm reminded of a trilogy by the British SpecFic writer Peter F. Hamilton; the Night's Dawn trilogy. In it, the dead are coming back by possessing the living, and as an incidental consequence of that whole "thing", can basically generate really, really high level Imachination illusions at will. One of the issues that comes up in the second book (wherein Al Capone comes back, and starts an interplanetary/interstellar empire consisting of a combination of the dead and the living, an accomplishment since the dead are afraid of the sky (and the dark, and space in general. It reminds them of the awful, awful afterlife). It is a series you pretty much need to read at some point.) is that, in a culture where people can, you know, create an unlimited amount of any material substance (pretty much, or near enough, at least), you can't use physical currency.
And so, they use entirely electronic banking.
But what to base it on, in this case, since physical substances can't be trusted? How about hours labored?
One way that this could be handled is having three "tiers" of monetary value, with a varying exchange rate set by a central bank in each culture.
You'd have the "base" hour, which would be the cost for one Baccalaureate principle preparation or an hour of common labor.
You'd then have the next step up, which would the cost for one Magisterial principle preparation or an hour of skilled labor.
The final step would be the equivalent of a single preparation of a Doctorate level principle; I can't think of a mundane equivalent.
The idea is that your amount of money under this system is closer to "I can exchange this much money for this much grammaristic stuff."
Alright, now how would you keep track of said money using Grammarie? There isn't a single method of storing cash that would be foolproof; well, there is one, but you'd need a 10th level Dreamason to do it (it involves overlapping a Zeitgeist, a Heuristic Net that covers an entire city, and personal Nets around everyone, and also requires that the nets can "talk" to each-other), and that's... infeasible, unless...
For this setting, what's the level range? You said that we're basing this off of Eberron, which would imply low-level stuff.
AAANNNND I just read up the thread a bit. So that method is completely infeasible.
Damn, I can only think of using Cursed Lead as a physical currency if you can't set up a network, since you can just have a very basic modification to, say, water (raise the freezing point to room temperature, and keep shop-rooms chilled).
Then, have the "coins" be little hollow cubes of Cursed Lead, with a grille on one side, and a "lid" on the other. This is pretty fine workmanship, anyway, so it would be pretty much the purview of the Gov.
To test it, drop your chunk of modified ice into the coin, and close the lid; this fills the compartment with an Antimagic field, thus causing the ice to slowly start melting; when you see ice on the other side of the grille, you know that it's a legitimate piece of "coinage."
Sure, you could con this with heated pieces of lead, but those wouldn't freeze again on the other side of the grille...
You'd have to heat the lead each time, to get all the water off, and then just let your water freeze back into a little chunk, and bam. An almost certifiable way to have currency in a world with unethical grammarists.