If a creature with evoke enters the battlefield without being cast, you won't have to sacrifice it.

Evoke is an alternative cost to cast the spell with evoke. It can't be combined with other alternative costs, such as casting a spell "without paying its mana cost."

The ability that causes you to sacrifice an evoked creature is a triggered ability. Players may respond to this triggered ability while the creature is still on the battlefield.

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