[Stephen_Zuckerman]

Not official, but I would say that it would be one contracting a mage to come set up the portal. Thus how much it costs, the permanent nature, and the level requirement (i.e. Uh... Who are you, again? Are you sure you can handle having a tunnel through space and time just laying about your home? Riiight... Come back and see me when you're a little drier behind the ears).