The only answer I could find
2,000 gp per lb.
This comes from A Magical Society: Silk Road, and while this may not be a first-party source from Wizards of the Coast, there does not seem to be any materials from them on the matter. I also could not find any other published third-party materials that list a price per pound for adamantine,to compare with.
If this value is too rich, or not rich enough, for you/your DM's taste, then read on as the rest of this answer will be dedicated to coming up with some alternative numbers and the calculations I have used to justify them.
Method 1: Average Item Weight
The first method is to find the cost of all items that can be made with the table that we are given in the Special Materials section, subtract the base cost of the item, divide by the item's weight, and average all these values of the same item type to come up with a value.
I performed this process both with and without the additional step of removing the item's masterwork cost before dividing by weight. The result was a range of values from 157 gp to 1,027 gp.
Method 2: Mithral Comparison #1
Looking at the prices for mithral we see that the price per pound is a half of light armour or shields, an eighth of medium armour, and an 18th of heavy armour. Applying these proportions to the values of adamantine items of these types gives us a range of 833 gp to 2,500 gp.
Method 3: Mithral Comparison #2
If we take the price per pound of mithral, multiply by the weight of an item that can be made of mithral, and then divide by the usual cost increase for that item we come up with a value which we will refer to as that item's Cost Efficiency Ratio (or CER).
For example, a mithral chainshirt:
\$CER = 500 \text{ gp/lb} \times 25 \text{ lb } / \text{ } 1000 \text{ gp}\$
\$CER = 12500 \text{ gp } / 1000 \text{ gp}\$
\$CER = 12.5\$
Assuming the CER is consistent between mithral and adamantine, we can rearrange the equation and change the mithral prices for adamantine ones to get the price per pound of adamantine for the item:
\$PPP = CER \times \text{light adamantine armour } / \text{ item weight}\$
\$PPP = 12.5 \times 5000 \text{ gp } / \text{ } 25 \text{ lb}\$
\$PPP = 62500 \text{ gp } / \text{ } 25 \text{ lb}\$
\$PPP = 2500 \text{ gp/lb}\$
Doing this for all armours and shields we can make from mithral gives us a range of 857 gp to 2,500 gp.
Method 4: Relative Masterwork Cost
This method was saved for last as it does not give a fixed value to work with, and requires there to be a masterwork version of the item you wish to create to compare costs with.
The additional cost of a masterwork weapon or ammunition is 300 gp and 6 gp, respectively. The additional cost of an adamantine weapon or ammunition, which includes masterworking, is 3,000 gp and 60 gp; ten times regular masterwork. This same comparison can be done for each of the armours, coming out at 33 and a third, 66 and two thirds, and 100 times masterwork for light, medium, and heavy respectively.
Therefore, for any item that can be made masterwork, you can choose whatever is deemed to be an appropriate multiplier, from 10 to 100, and simply multiply the masterwork cost by this value to determine its cost to make an adamantine version.
For example: thieves' tools normally cost 30 gp but masterwork ones cost 100 gp, a difference of 70 gp. By this method adamantine theives' tools would cost somewhere from 730 gp to 7,030 gp.
Method 5: Reclassification
While researching for answering this question I stumbled across the adamantine shears in the Pathfinder SRD. These are basically hedge trimmers with adamantine blades. They are described as being able to cut through things like a scimitar, which would explain their 3,000 gp increase over their regular steel counterpart.
Thus, this method is simply to ask yourself "does adding adamantine to this tool make it a weapon, armour, shield or ammunition?" and if it does, simply use the apporpriate price.
