Targath is much rarer (which would make it more expensive) and has much less uses (which decreases the price back again). So, since it is rarer it is not suited to coins. Besides, the reason for using copper, silver and gold will lead back to tradition when the Eberron society was much more like a normal medieval world. Mining technology will have been different then and Targath much rarer.
Tradition has no place in a world were the coins are only worth their weight. If you can be reasonably sure to fetch the same price anywhere you trade with your Targath. And that price is the same as silver, there is no reason not to carry some Targath. Especially if it weighs less.
Also, even though it is a lot rarer it is still worth the same, there is no reason you couldn't just as well have some targath coins. Since it is rarer it would only be worth more if minted, making it very suitable for coins.
I did the same calculation, but on a breastplate and weight of 5kg (besides, a full plate weights closer to 30 kg than 10- my winter jacket weights 3.5 alone Wink ) and those added up nicely, and give the current prices.
Medium armor +10,000 gp
So, 5 kg of adamantine is worth 3.333 gp (one third of 10.000 for materials) = 666 gp per kg. Since each coin is 10 g (they say it weights 1/3 of an ounce ~10 g). that is 6660 g (6.6 kg) for 5 kg adamantine. So in this case adamantine is the cheaper.
But this probably arises from the fact that they have made those categories, plus we cannot really generalise and say 1/3 of materials for both a breastplade and a full plate, as a full plate will require much more work to fit the parts together.
I'll make a double check when I'm a little more energitized (just got home from reenactment).
Here's another way to look at it from the D&D pricing perspecitive:
Mithral actually has a price per lb, it's 500gp/lb. Gold costs 50gp/lb. This is 10 times the price of gold, now that Mithral might be worked, giving it a cost of 3.3 times the cost of gold unworked. Adamantine seems to at the least cost more than Mithral. From 5 times the cost in a light armour, to 5/3 the cost in heavy armour. At the very least estimate, adamantine is priced 5 times the price of gold from this estimate.
Now, since your players don't have a lot of cash, you could make it cheaper than that. That would help tremendously with us getting better equipment :wink:
1. Just because the Romans did it, doesn't mean it should be introduced in a medieval setting The romans did it, but from 400-1300 economies were almost solely based on natural value. Even after 1300 the way towards representational economy was very long and arduous indeed. In Eberron you can see the beginnings of the system in the letters of credit - how long before you can trade those as representational money?
The economies from 400-1300 were probably based on natural value due to a limit on what you could trade. When you can trade just about anything in big cities. Coins would probably either have to be intrinsic, or fall out of fashion fast.
Imagine you come into Sharn with 5 kgs of adamantine, a man offers to pay you standard price for it. Lets assume this is 10 kgs of gold. Why would you want to take his trade? You could just as well on any other day go to one of the 100k citizens with this kind of money and sell him some Adamantine. All this would solve you would that you can easier use the money piecemeal. But you would still be down from the deal, even though it is the standard fare you are now travelling with 10kgs of stuff, and you can probably only trade your gold down in value. Adamantine on the other hand could be traded up in value.
On the other hand, imagine you are in a bunch of small town, tech level year 800. You have the same 5kgs of adamantine. Somebody offers you 10kgs of gold for it. You take that offer, because you have no idea how long you have to go before you can get your bar of adamantine traded. And your gold can be used in smaller portions.
So my point is: In a large non-intrinsic economy anyone who isn't a complete wacko would take whatever exotic good (be it targath, fruits, spices or adamantine) you came and attempted to bargain with. He would probably be able to trade it on for a small profit to someone who needed it and thus getting extra money for just doing buisiness with you. Even if he couldn't get a profit from it, it would be as good as money in the bank, since trading it on would be little problem, there are enough people around town that somebody needs it and can pay.
In a small non-intrinsic economy (the only place where it actually makes sense, like europe from 400-1300), he wouldn't be taking your spices (or whatever) as valid payment if he didn't know anyone within 20 miles who needed spices. He probably still would barter for it if he could fetch a good price.
In an intrinsic economy, coins are the most sought after commodity (partly due to their security in trade, but moreso because the government says it is). Which means you would only accept something instead of coin if you could sell it for more coin with relative ease.