If they really are emerald ores, you could pretty easily restrict the biome generation algorithm to only create extreme hills when there is money in the bank. I

If they really are emerald ores, you could pretty easily restrict the biome generation algorithm to only create extreme hills when there is money in the bank. I expect the "mobs are stronger the further away from spawn" is also meant to put a bit of a dampener on the rate you can grab them.