Does Cascade look at CMC or mana cost payed?
Cascade checks the converted mana cost of the spell, which doesn't include any additional costs or cost reductions.

The definition of Cascade is in rule 702.84a:


  Cascade is a triggered ability that functions only while the spell with cascade is on the stack. “Cascade” means “When you cast this spell, exile cards from the top of your library until you exile a nonland card whose converted mana cost is less than this spell’s converted mana cost. You may cast that card without paying its mana cost. Then put all cards exiled this way that weren’t cast on the bottom of your library in a random order.”