If I give a unit deflect in response to a spell (like Rebuke) that targets it, does the deflect cost apply to that spell?

Ruling: No, giving a unit deflect in response to a spell does not apply to that spell. Deflect only applies if the unit has deflect while a spell or ability is being finalized (choosing targets and paying costs).

Sequence:

• A spell is finalized (targets chosen and costs paid)
• The spell becomes a legal chain item that can be reacted to
• By the time you can react to the spell, finalization has already ended
• Deflect given at this point cannot retroactively increase the spell's cost

Nuances:

• The finalization process ends when the spell becomes a legal chain item that can be reacted to
• There is no way to give a unit deflect in response to a spell that would make that spell cost more