Free within 5 minutes of booking, and at the moment there is no cancellation fee after that either. The Cancel ride? sheet says what cancelling costs before you tap a reason, so a fee set later is always shown first, and your first fee in any 30 days is waived. A fee not taken from a card hold must be paid before your next booking.
What does it cost to cancel?
Free within 5 minutes of booking, and no cancellation fee at the moment; the app shows any fee first.