The protection stones can only go as low as they can currently. Think about this:
Some lowbie (a griefer, let's say) walks along and notices that Diamond Protection is cheaper. They realize what they can do with the PStone and mine a few iron and purchase one. They place it somewhere just to screw over the rest of the people on the server within those protected blocks.
So, if we made them any cheaper, they can potentially be used against the rest of the players, and that is what we want to prevent.
