Cut of the Profits

X will be the same for both the original spell and the copy you create with the casualty ability.

Casualty N means “As an additional cost to cast this spell, you may sacrifice a creature with power N or greater.” and “When you cast this spell, if a casualty cost was paid for it, copy it. If the spell has any targets, you may choose new targets for the copy.”

If you pay the casualty cost of a spell, the copy will resolve before the original spell.