a lack of supply drives up price
Which is a good thing...for those who are selling the stuff. Consequently, the only people selling will be the ones with the diamonds in the first place, since the only way to get to the nether is through an obsidian gate. Only the people who already have nice things will profit off of this. The richer get richer, those who want to look rich get poorer.
Good people of Minerealm, cast off the shackles of the oppressive bourgeois! WORKERS OF THE REALMS, UNITE!
I know I'm blowing this out of proportion, but there are plenty of contested resources in the regular map. The danger of going to the Nether is enough of a payment to get the stuff. Just because the resources are there doesn't mean that they will become common.