In this article it is proven that no cross-monotonic cost-sharing method can guarantee a recovery of more than 12 of the cost of the computed tree in the Steiner Tree game. Hence the algorithms described by Jain and Vazirani  (Steiner Tree game) and by K¨onemann et al.  (Steiner Forest game) are the best possible. This is accomplished by modifying an instance used in the paper by Immorlica et al.  for the Facility Location game, and bounding the expected value of the sum of the cost shares.