Interesting question:
I recently booked a flight FWA-DTW-SEA in Y. Cost of the ticket was ~$450. A detailed breakdown of the cost (for calculating sky miles), showed that the FWA-DTW portion of the flight was only $9, and that most of the cost was on the DTW-SEA leg, with the remainder going to taxes.

Does that mean that if DL wanted to IDB me for the FWA-DTW leg on the way home, I would legally only be owed $36 (4x$9) because that is what 400% of the ticket price was?