I have no idea what magical fantasy flights you're booking, but United charges $200/$400, Delta offers a $200/$450 rescheduling fee, Alaska's at $175 and Virgin is at $100 (which, since they don't do round trip, means it's actually $200).
I had a $159 ticket from Los Angeles to Seattle. I was going to help my uncle haul a trailer. Then the person who needed the trailer hauled backed out. So I called up Alaska to cancel my ticket.
They tried to charge me $16.
To the best of my knowledge, all of the above require you to book directly with them and don't entertain third party tickets.