Historical Rules · 17 July 2015
Rule 702.65a
Official source text from the 17 July 2015 Rules release
Delve is a static ability that functions while the spell with delve is on the stack. “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.