Casting Eidolon of the Great Revel doesn't cause its own ability to trigger. It must be on the battlefield when the spell is cast for its ability to trigger.

The amount of mana spent to cast a spell doesn't change its mana value. For example, a card cast with its morph ability has a mana value of 0 even though was paid.

The mana value of a spell with in its mana cost includes the value chosen for X.