Costs — Riftbound Rules

202.

Some Game Effects will instruct players to pay resources, spend counters or XP, or perform game actions as a Cost in order to perform a linked Effect.

203.

Costs must be paid in order to get their linked Effect.

203.1.

Paying a Cost requires a player to perform the appropriate game action: pay resources, spend counters or XP, kill units, etc.

203.2.

The Cost is still considered paid even if the associated game action is replaced.

203.3.

If the game action associated with a Cost is impossible for any reason such that a player cannot perform it, then they cannot pay the Cost and they will not execute the linked Effect.

Related FAQ (2)

What costs does Ezreal, Prodigy reduce? Does he reduce Repeat costs? Accelerate costs? Equip costs?

Ezreal reduces optional additional costs using both "additional cost" and "may." Repeat and Accelerate qualify, but not Equip costs - that's an activation cost for an ability, so it's neither optional nor additional.

What about Deflect costs? Aren't those optional now?

No. Deflect is a mandatory additional cost, meaning you must pay it if you want to finalize a spell or ability that chooses something with Deflect.