The buy back program when I signed up for it in April (they may call it something different) was you pay a fee, like $30, and if you trade it in within 18 months you'll get 50% of the original off-contract price for it. So a phone that is $700 off contract will get you $350. If you wait until you're eligible for a new upgrade, like 21 months in or whatever it is, you get 30% of the price and if you wait until 24 months you get 20%... or something along those lines.
But yeah, my Evo 3D was a p.o.s. and after 3 of them I just sat it out with my broken one (internals, not hardware issues) for a few months until Sprint released the Nexus. I had one tiny, like 5 mm, scratch on it so they gave me like $45 for it when I didn't use the buy back program. If I had the buy back program, I'd have gotten a couple hundred