Ghalta, Primal Hunger

• WWizards of the Coast2018-01-19
  Official Ruling
  To determine Ghalta's total cost, start with the mana cost (or an alternative cost if another card's effect allows you to pay one instead), add any cost increases, then apply any cost reductions. Ghalta's mana value remains unchanged, no matter what the total cost to cast it was.
• WWizards of the Coast2018-01-19
  Official Ruling
  If a creature's power is somehow less than 0, it subtracts from the total power of your other creatures. If the total power of your creatures is 0 or less, Ghalta's cost remains .
• WWizards of the Coast2018-01-19
  Official Ruling
  The total cost to cast Ghalta is locked in before you pay that cost. For example, if you control three 2/2 creatures, including one you can sacrifice to add , the total cost of Ghalta is . Then you can sacrifice the creature when you activate mana abilities just before paying the cost.
• WWizards of the Coast2018-01-19
  Official Ruling
  Ghalta's first ability can't reduce its cost below .

Rulings via Scryfall