Rule 702.51b

The convoke ability isn’t an additional or alternative cost and applies only after the total cost of the spell with convoke is determined. Example: Heartless Summoning says, in part, “Creature spells you cast cost {2} less to cast.” You control Heartless Summoning and cast Siege Wurm, a spell with convoke that costs {5}{G}{G}. The total cost to cast Siege Wurm is {3}{G}{G}. After activating mana abilities, you pay that total cost. You may tap up to two green creatures and up to three other creatures to pay that cost, and the remainder is paid with mana.