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.

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.

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.

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.