357.2.a. Costs that are replaced with other events by replacement effects are still considered paid. Example: A player plays Cruel Patron, which says "As an additional cost to play me, kill a friendly unit." They also control Zhonya's Hourglass, which says "If a friendly unit would die, kill this instead. Heal that unit, exhaust it, and recall it." They choose to kill a friendly unit during step 3, but as they pay the cost in step 4, Zhonya's Hourglass replaces that unit's death. The cost is considered paid, and the player can continue playing Cruel Patron.