Rule 356.4.c

Discounts that only apply to a component of the cost will be applied when that component is added to the cost of the spell and before any other discounts. Example: Ezreal, Prodigy reads “optional additional costs you pay cost [1] or [A] less.” When playing a Frigid Touch and choosing to pay the additional cost in step 2, as soon as the additional cost is added to the cost of the spell, Ezreal, Prodigy’s discount is applied to it.