Spirit Airlines doesn't include bags in its base allowance (Bare Fare). Additionally, baggage prices rise the closer you get from take-off. Your best option is to purchase your allowance at the time of your first booking.
Another thing to consider: Spirit's baggage policy stipulates that the maximum bag weight is 40 pounds (18 kilograms). This is 10 lb (5 kg) less than the average and you can be sure you will pay more for it.
