The Skullspore Nexus

This spell costs {X} less to cast, where X is the greatest power among creatures you control.
Whenever one or more nontoken creatures you control die, create a green Fungus Dinosaur creature token with base power and toughness each equal to the total power of those creatures.
{2}, {T}: Double target creature's power until end of turn.

The Lost Caverns of Ixalan Promos

The Lost Caverns of Ixalan

Once you determine the cost to cast The Skullspore Nexus, you may activate abilities to pay that cost. If the greatest power among creatures you control changes while activating mana abilities, the cost to cast The Skullspore Nexus 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 (for example, if you control a creature with "This creature's power is equal to the number of cards in your hand"), that new power will be used to determine the cost reduction.

To double a creature's power, that creature gets +X/+0, where X is that creature's power as The Skullspore Nexus's activated ability resolves.

Use the power of the nontoken creatures that died as they last existed on the battlefield to determine the base power and toughness of the token created by the middle ability.