Kaza, Roil Chaser

• WWizards of the Coast2020-09-25
  Official Ruling
  To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Kaza's activated ability). The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.
• WWizards of the Coast2020-09-25
  Official Ruling
  Kaza's cost reduction can reduce only generic mana in a spell's cost. It can't reduce colored mana requirements.

Rulings via Scryfall