Personally, I don't think market forces are or should be relevant in this case. Which is to say, for identify you need a white, AAA, 7mm pearl. Now, that might cost 50gp from the halfling pearl divers at the coast. Or, 100gp from the pearl and jade exchange in the market. Or, 200gp a month's journey inland. But if it's not AAA (95% free of defect) and 7mm in diameter it won't work. A steel grey pearl might help you identify metallic items easier, however.
For something like raise dead, it might not be a diamond, but a 500-1000gp tithe to the temple. Or, 500gp to have the temple set up the ritual go on a mystical journey into the underworld. You recognize Apil-Shadad from the brewery playing the role of the Dread Gatekeeper at the cave entrance, but as you go further in things become more mystical. If successful, you find where the priests hid the body, and, if the gods favor you in this liminal space, your friend stands up and walks out with you.
It strikes me that the key part isn't the diamond, but something appropriately mythic ("diamonds are forever"), with a non-trivial cost, that requires some forethought if you want to do the ritual "in the field" as it were.
Or, this is a feature and not a bug. Be careful and don't die.