cheap prepaid mobile plan for 3 months

I will be staying in the US for 3-4 months and need a sim for my unlocked iphone 5.

At home I play $12 for 500mb of web data and 40min free calling.

I am looking for something similar, as a prepaid/pay-as-you-go plan. (yes I know it will be a lot more expensive in the US)

I looked at t-mobile, they seem to offer a plan for $30 for 5gb web and 100min but only for devices bought at wallmart (???) and other plans start at $50.

at&t seems to have only $60+ plans..

Any recommendations?