They Buyback program I am sure is not full price. Yea, you buy a phone for $200 and a year later you bring it back and they offer you $20, sure they WILL buy it back but, for what cost ? This is for people who don't want to deal with trying to sell a old device.
You'll always get more for it selling it outright, Just think of trading in a car, it's just about the same I would assume.
Edit:
Link...$200 Phone 24months later (after your 2 year contract) worth $40 (20% of it's price), not the best deal in the world...
http://www.bestbuy.com/site/Misc/Buy...at230000050010