Does cascade let you cast the revealed card for free or do you still pay its mana cost?

Cascade is a triggered ability that triggers when you cast a spell. You exile cards from the top of your library until you exile a nonland card whose mana value (converted mana cost) is strictly less than the cascade spell's mana value. You may then cast that exiled card without paying its mana cost. All other exiled cards are put on the bottom of your library in a random order (CR 702.84c).

Casting without paying the mana cost is a powerful benefit: you skip the normal payment step entirely. However, if the card has an alternative cost or additional costs (like kicker), you may still choose to pay those — but you are never required to pay the card's mana cost (CR 117.9a).

You are not forced to cast the revealed card; the ability says 'you may.' If you decline, the card is simply shuffled to the bottom along with the others (CR 702.84c).

Example: You cast Bloodbraid Elf (mana value 4) and cascade triggers. You flip cards until you hit Lightning Bolt (mana value 1, which is less than 4). You cast Lightning Bolt dealing 3 damage to a creature — paying zero mana for it. The remaining exiled cards go to the bottom of your library randomly.