Does Ezreal - Prodigy's cost reduction apply to Irelia - Legend's optional power payment to ready a unit?

Ruling: No, Ezreal - Prodigy does not reduce Irelia - Legend's power payment. Additional costs only refer to effects that specifically use the term "additional cost" in their text.

Nuances:

• The primary examples of additional costs are Repeat and Accelerate keywords
• Some individual cards may also have additional costs explicitly stated
• Optional payments (like Irelia's "may" ability) are not the same as additional costs