This is a possibility:
Since the FEP prefix is older, no more bricks in that vicinity are being found. Thus, the world supply of the FEP is likely static at the low population that it had early on.
If I understand correctly, FEP was found to be very scarce. And now that no more will be appearing (You would have to get a brick of FEP or FER or FES or maybe FET???), the supply is capped.
Similar to the HPA insert. There is a slim chance that more may be found, but I emphasize SLIM.