The cost reduction applies only to generic mana in the cost of creature spells with flying you cast. For example, if you cast a second Watcher of the Spheres, its cost won't be reduced below .

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Watcher of the Spheres). The mana value of the spell remains unchanged, no matter what the total cost to cast it was.

A creature spell that doesn't have flying won't cost less even if an effect will cause the creature to have flying once on the battlefield. This is also true if the creature spell itself has an ability that gives it flying once on the battlefield under certain conditions, even if those conditions are true.