In the SRD, under Special Materials, under Mithril, it states that - for other items - the cost is 500 gp per pound.
Interestingly enough, it does not list such for adamantine.
Hmm, one pound of gold is 50 gp, which equals 5 pp. So, if you make mithril coins following the core procedure of 50 coins per pound, regardless of weight or size, then 1 pound makes for 10 mithril coins, meaning that one mithril coin is equal to two platinum coins. Does that make Mithril the electrum version of platinum and some other metal?

Of course, this also suggests that it weighs a
bit more than its entry would have you think . . . .
Adamantium seems to be about 2 to 5 times costlier (or heavier, which with metals is more or less the same thing due to the core standard for coins) than Mithril. So perhaps simply presume it to be five times more expensive - thus keeping the electrum idea in play.*
* (Electrum is fivefold the value of silver but half the value of gold. Mitrhil seems to be twice the value of platinum but perhaps a fifth the value of adamantine.)
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Of course, the real problem is that adamantine cost seems to be arithmetic (5k, 10k, 15k, etc), whereas mithril cost is based on a power rule (1k, 4k, 9k, etc). The 500 gp per pound is a nice guideline, and I tend to think the cost of an item made of it - whether shield or armor or arms - should be based on this rule instead of a vague price "standard." Thus the dagger and the greatsword made of mithril should have quite different costs, let alone a suit of full plate!
Pperhaps we can extrapolate from the following:
5/01 = 5.000
10/4 = 2.500
15/9 = 1.667
This follows a curve, unfortunately, but if we average the three we get (10 / 4.667 ~ 2).
So, while it is not RAW, it would seem that 1000 gp per pound may be a good baseline for adamantine materials (ie: double mithril costs). That said, just chuck out the costs for weapons, armors, etc made of it and instead base it on the weight of the material (presuming it is composed entirely of the metal, of course).