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.
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 {10}{G}{G}.
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 {C}, the total cost of Ghalta is {4}{G}{G}. Then you can sacrifice the creature when you activate mana abilities just before paying the cost.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Johann Bodin
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Ghalta's first ability can't reduce its cost below {G}{G}.
