Ghalta, la Inamovible

Ghalta the Immovable's cost reduction ability can't reduce the total cost to cast the spell below {W}.

Ghalta the Immovable's last ability doesn't actually change any creature's power. It changes only the amount of combat damage the creature assigns. All other rules and effects that check power or toughness use the real values, even if they cause damage "equal to a creature's power" to be dealt.

Ghalta the Immovable's mana value doesn't change no matter what the greatest toughness 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 toughness of creatures you control.

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

The first step of casting a spell is to move it to the stack. If this causes the greatest toughness of creatures you control to change (perhaps because you control a creature whose toughness is determined by the number of cards in your hand), that new toughness will be used to determine the cost reduction.