Does a God's own mana cost count towards its own devotion?
To quote one of the rulings on Gatherer:


  If a God enters the battlefield, your devotion to its color (including
  the mana symbols in the mana cost of the God itself) will determine if
  a creature entered the battlefield or not, for abilities that trigger
  whenever a creature enters the battlefield.


Also from the rules:


  700.5. A player’s devotion to [color] is equal to the number of mana symbols of that color among the mana costs of permanents that player
  controls. A player’s devotion to [color 1] and [color 2] is equal to
  the number of mana symbols among the mana costs of permanents that
  player controls that are [color 1], [color 2], or both colors.


Since the god is a permenant, it's mana symbols in it's casting cost count towards your devotion.