Mulldrifter — ruling 1

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

Mulldrifter — ruling 2

If you pay the evoke cost, you can have Mulldrifter's own triggered ability resolve before the evoke triggered ability. You can cast spells after that ability resolves but before you have to sacrifice Mulldrifter.

Mulldrifter : règles, fonctionnement et interactions (MTG)

Mulldrifter is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flying When this creature enters, draw two cards. Evoke (You may cast this spell for its evoke cost. If you do, it's sacrificed when it enters.)

How it works : Mulldrifter

Original English text — Wizards of the Coast / Scryfall

#1March 19, 2021· Wizards of the Coast
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as an evoke cost), add any cost increases, then apply any cost reductions. The mana value of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was.
No other card is named in this ruling.
#2March 19, 2021· Wizards of the Coast
If you pay the evoke cost, you can have Mulldrifter's own triggered ability resolve before the evoke triggered ability. You can cast spells after that ability resolves but before you have to sacrifice Mulldrifter.
No other card is named in this ruling.