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.

The last ability of Warden of Evos Isle can't reduce the colored mana requirement of a creature spell with flying.

To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.