The Great Henge

Once you determine the cost to cast The Great Henge, you may activate mana abilities to pay that cost. If the greatest power among creatures you control changes while activating mana abilities, the cost to cast The Great Henge remains what you previously determined.

The first step of casting a spell is to move it to the stack. If this causes the greatest power among creatures you control to change, that new power will be used to determine the cost reduction.

Once The Great Henge's last ability has triggered, you'll draw a card even if you can't put a +1/+1 counter on the creature for some reason (most likely because it has left the battlefield).