Defy

Q. You play Rocket barrage repeated. Can your opponent play Defy and counter it, given you paid 8 En,ergy and 2 Power to play the spell?

Yes. Discounts, taxes and additional costs only affect when you are playing the card. Once the spell is on the chain, all that information is gone!

130.4: Effects that need to determine a card's cost for any purpose always use its base cost, even if that cost is altered or ignored as the card is played.