Teval, arbitre de la vertu

If a spell on the stack has an {X} in its mana cost, use the value of X chosen as it was cast to determine that spell’s mana value.

“Delve” means “For each generic mana in this spell’s total cost, you may exile a card from your graveyard rather than pay that mana.” The delve ability isn’t an additional or alternative cost and applies only after the total cost of the spell with delve is determined.