Rule 356.4.g
If a discount applies a variable cost reduction to the card, that value is checked now. Example: Rhasa, the Sunderer reads “I cost [1] less for each card in your trash.” The value of this discount depends on the number of cards in the trash. That value is checked during this step when the discount is applied to Rhasa’s cost. If the number of cards in his controller’s trash changes at a later point in the finalization process, that will not affect the value checked during this step or the total cost calculated in this step.