Various cultures of Ixalan tell stories of the monsters of legend that ended up as gristle between Ghalta's teeth.

Ghalta, Primal Hunger

This spell costs {X} less to cast, where X is the total power of creatures you control.
Trample (This creature can deal excess combat damage to the player or planeswalker it's attacking.)

Foundations Jumpstart

She is the predator's appetite, the hunter's ferocity, the primordial progenitor of claws and fangs.

Foundations

This spell costs {X} less to cast, where X is the total power of creatures you control.
Trample

Foundations Promos

Ghalta's first ability can't reduce its cost below {G}{G}.

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.

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.