I just got a OnePlus One from Swappa for $230. Went with Cricket Wireless on the $40 plan. -$5/month for auto credit card payments. Rooted the phone and all it good.
If you know you are going ATT and just need a smart phone, I think I have a spare iphone 4s around here somewhere I could be convinced to let go fairly cheaply. Its still carrier locked to ATT, but if you are going with ATT, no problem. (Its only a problem if you later want to go to another carrier like StraightTalk)