I never got this. If the Sphinx isn't supposed to be an 'immobile' super combatant, the teleporting rock par excellence, what on earth is the justification for its high cost? You don't surely plan a strategy that relies on having your home citadel stormed on a regular basis.
If we are to have two categories of immobile pretenders, those that can teleport around but not move on the battelfield or by normal tactical movement, and those that are totally rooted to one spot, it would make more sense for the cheaper ones to be rooted to the spot. And there is more of a thematic justification for pretenders like the Fountain of Blood or the Oracle to be rooted to the spot, as they are presumably reliant on adequate plumbing which cannot be teleported along with them!
For that matter, it could be cool have a pretender that was fully mobile on the battlefield, but could never leave his/her home province. Like a Genius Loci or similair spirit tied to a place.