i have been researching this a lot lately and the answer is easy yet complicated. all plans will give you unlimited talk and text (or at least i have not found any plan on any US carrier that does not).
since you asked about att, it will be 25 a month for the talk/text + data ($20-100) + the phone cost = $65 + data cost.
if you have the cash on hand, you can buy the phone upfront and there are some cheaper plans from other carriers. for instance boost mobile has $100 off retail if you buy the phone all at once $750 for the one you want and has $45 a month for 5 GB high speed data or $55 a month for 10 GB plus unlimited talk/text total price (this amount of data is $75 or $100 a month with ATT!!!! plus talk cost) this assumes you have the 750 up front for the phone.
the best price i have seen allowing you to pay for the phone in payments is sprint (boost mobile is actually part of sprint). $50 a month for unlimited talk/text/data plus the phone payment which of around 30 is roughly 80 a month no contract and something like $50 down.
some basic info for you. hope it helps you some. have read a lot for myself and thought i'd share some of what i have found. i ordered the boost phone yesterday and as a side note, the boost plans are sale prices good until nov 3 so got a couple weeks to work that angle