There are a few things I would like to know from you.
1) Are you coming to USA for a short visit?
2)Where you buy your Samsung Galaxy phone from??
3)Are you using lots of data?? AT&T do have a unlimited Gophone monthly plan for US$60,with 2.5 GB of data inc.See here.Smartphone plans.
http://www.att.com/shop/wireless/plans/prepaidplans.html