Ghalta the Unstoppable

Once you determine the cost to cast Ghalta the Unstoppable, you may activate mana abilities to pay that cost. If the greatest power of creatures you control changes while activating mana abilities, the cost to cast Ghalta remains what you previously determined.

Ghalta the Unstoppable's mana value doesn't change no matter what the greatest power of creatures you control is.

Once you announce you're casting a spell, no player may take actions until the spell has been paid for. Notably, opponents can't try to reduce the power of creatures you control.