Offalsnout

Effects that cause you to pay more or less to cast a spell will cause you to pay that much more or less while casting it for its evoke cost, too. That's because they affect the total cost of the spell, not its mana cost.

Evoke doesn't change the timing of when you can cast the creature that has it. If you could cast that creature spell only when you could cast a sorcery, the same is true for cast it with evoke.

Whether evoke's sacrifice ability triggers when the creature enters depends on whether the spell's controller chose to pay the evoke cost, not whether they actually paid it (if it was reduced or otherwise altered by another ability, for example).