It would be cool to be able to change the cost of opening the portal to either be really cheap or really expensive.

Hm, I think I thought about this a while ago (actually something similar, being able to change the elven portal multiblock), but decided against it because it's too hard to communicate to players. I don't want to cause Botania's bugtracker to fill up with "my portal isn't opening, works fine in other modpacks"

A tooltip could be made on the elven gateway core item (by the pack maker or you) as well as communicating through the quest book. Or maybe a message in chat when the portal is attempted to be lit with insufficient mana along the lines of "the amount of mana needed has been tweaked in this pack"