Prismari Pianist

For spells on the stack with {X} in their mana costs, use the value chosen for X to determine the spell's mana value.

A spell's mana value is determined only by its mana cost. Ignore any alternative costs, additional costs, cost increases, or cost reductions.

Prismari Pianist's ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered or otherwise leaves the stack without resolving.