Take that back. I was thinking of Vanish. Invis is 4500. Ok i do suppose i see the massive limit on usage, but not once a day? I my mind the item functions like, "you walk in to the king and see he is standing with someone. He glances over and the stolen armor gives him no reason to suspect you. You walk up to present the documents from the guard reports. Slight of hand, stab the dignitary. As the poison begins to hit his blood stream, you turn and run activating snapleaf."
Well as an assassin, you don't do this once in a while, in fact this is a known tactic by the worlds most successful and elusive assassin. You do this daily (or for 750gp, weekly)
One time use seems steep but i could understand a once in a week use or something.