Can you use Defy to counter Bullet Time, and does the variable power cost affect whether Defy can target it?

Ruling: Yes, you can Defy Bullet Time. Defy only looks at the printed energy and power cost in the upper left corner of the card, not any additional costs paid during resolution.

Sequence:

• Bullet Time is cast and goes on the chain
• Defy can target it based on its printed cost (0 energy, 0 power)
• If Defy is used, Bullet Time is countered and the caster doesn't have to recycle any power
• If Bullet Time resolves without being countered, then the caster pays power during resolution (at which point it's too late to Defy)

Nuances:

• The "pay any amount of power" text on Bullet Time is not a cost, it's part of what the card does during resolution
• Defy always cares about the printed cost on the card, not modified costs (this also applies to interactions like Deflect)