I don't know about AT&T, but at Verizon, you can buy one of two data plans. A 250MB plan for $40 and a 5GB plan for $60. I just use the cheaper plan, and if I plan on traveleing somewhere where I might be forced to use Verizon's network, I can upgrade to the 5 GB plan for a month and then go back to the cheaper plan the next month.
I went to AT&T's website and they have a $40 200MB plan and a $60 5 GB plan. They don't list the Nokia as one of their devices yet, but it seems unlikely that the only available plan will be the $60 one. So you ought to be able to use the cheaper plan most of the time except for those ocassions when you know that you might need more data.