My reading of BIND MAGIC is that the additional WP is spent on the actual spell that is being bound rather than the BIND MAGIC cast: Otherwise, the "...don't count toward the Power Level" rule doesn't make any sense given that the Power Level of BIND MAGIC itself is irrelevant as far as I can tell.Bind Magic scales with the amount of Willpower spent with the highest level creating a reusable item.
Of course, this is my own reading of the spell, which might easily be mistaken...