I believe on a basic economy fare, if you cancel more than 24 hours after booking, you only get a partial travel credit. They deduct $99 on domestic round-trips and $199 on international round-trips. So you can pay $300 to save $199 (if you need to cancel), but you'd really only do that if you currently intend to fly and want the other benefits (seat assignments, upgrade potential, I think more mileage earning).