When you set this scheme in motion, you may pay. If you do, put each nonland permanent target player controls with converted mana cost X or less on the bottom of its owner's library.
6/15/2010 You choose the target player when the ability triggers. As the ability resolves, you choose a value for X and decide whether to pay . If you do decide to pay , it’s too late for any player to respond since the ability is already in the midst of resolving.