If you Repeat a spell like Missile Barrage (which costs 4 Energy + 1 Mind), does the total cost become 8 Energy + 2 Mind for the purpose of cards like Defy that check cost?

Ruling: No. Effects that check a card's cost always use its base (printed) cost, not the total cost paid. Defy would still be able to counter a Repeated Missile Barrage because it checks the base cost of 4 Energy + 1 Mind.

Nuances:

• This principle applies to all cards that look at costs - they always reference the printed cost, not what was actually paid