Partial Refund of a non-refundable ticket
Not sure if you know this but... At least United (maybe others) will give you back money (in the form of travel credits) for a flight that you book, where the prices drops after you book it.
Example: Denver to St. Louis around XMAS. Booked it at $317. Frontier Airlines added the route and forced the price down to $213. United price matched. Called United. Travel credit coupons to be mailed in 3-4 weeks.