The mana cost of the creatures being cast is still the stated cost on the card, even though you did not pay the cost.

You can’t use Aluren when casting a creature using another alternate means, such as the Morph ability.

You can’t choose to cast a creature as though it had flash via Aluren and still pay the mana cost. You either cast the creature normally, or via Aluren without paying the mana cost.

Aluren checks the actual printed cost on the creature card, and is not affected by things which allow you to cast the spell for less.

If creature with X in its cost is cast this way, X can only be 0.