I bought a AT&T SIM from a GZ subscriber a few weeks back and have now done my trip to the USA with no immediate return planned.  So I have for sale, a AT&T Go Phone SIM, which has been activated and a $40 plan (500MB, 500 minutes, unlimited text) applied - this lasts for a month so has about two weeks left to run.  Unfortunately I can't check usage (ATT send a password by SMS but roaming is not enabled) but I would estimate I used no more than 200MB and 10 minutes.
SIMs are $10 in USA normally so it's going to cost you $50US ($62 NZ) to set this up if you buy new
Open to offers but looking for about $40 inc postage