Adamantine is supposed to be a skymetal, from meteors that fell to Golarion (or wherever). It isn't supposed to be something you can mine in the zillions unless you're in the elemental plane of earth, perhaps.
Consider what you want the story to be about. If you want to provide lots of adamantine as a plot point in your story, have the Drow discover a large adamantine meteor that has been long buried.
If you'd rather keep the mine aspect, change it to mithral.
Now, we don't have a price for either adamantine or mithral ingots, but the cost of a general item made of Mithral is 500gp/lb (pre-conversion weight). The cost of materials for crafting is 1/3 of final cost, and the whole cost of materials won't be only Mithral, so let's say that it's 160gp per lb of mithral.
The difference in cost between a given item made of adamantine, versus one of mithral, varies from 5 times to about 1 and 2/3 times the price, but seems average out to about 3 times the price. (There is no price given per pound, so we have to guess at it.) Let's say 500/lb. This doesn't square with the price of an adamantine weapon, such as a greatsword, but we can suppose that being made into a weapon or armor requires alloying with steel or other metals that make the price less sensitive to the actual amount of adamantine in the finished product.
So, let's say 500gp/lb. This fits with an extremely rare metal only available in meteors that is in very high demand for weapons and armor.