The Fitbit Inspire 2 and Fitbit Ace 2 are the cheapest available devices at $69. The Fitbit Inspire 2 is a slim, lightweight tracker with 24/7 heart rate tracking and strong fitness features. The Ace 2 is the fitness tracker designed for kids, so you probably don't want to pinch pennies there.
Additionally, the cheapest smartwatch on the list is the Fitbit Versa 2, the predecessor to the Versa 3. You'll pay a full $50 less than the new model but you'll lose built-in GPS and Fitbit's improved heart rate sensor, so choose wisely.