I have my son on Straight Talk. He has an AT&T compatible SIM.
He had been using a Motorola Atrix HD. He began having issues with the touch screen.
We took it to a local repair store that also sold used phones. They had an HTC One X for $220. They gave me $80 credit for the Atrix.
My point is, used smartphones hold their value well. Especially the more popular ones.
This is why a lot of people get new phones every 6-12 months. This is how I do it.