I did it for free once, but I think the airline was just bad at math. I flew most of the legs of my flight. Then the last leg a hurricane showed up and they offered me an opportunity to rebook since it was likely to cancel the flight.
When I rebooked, the airline gave me a credit for the round trip flight in total. I only had to book a one way ticket on the last leg, so I obviously was able to "afford" the flight without additional expenditure on my part.
