It depends on your phone. I presume you have a GSM unlocked device? (maybe give us the model, so we can look up the bands)
I'm not American, so I might not have the best suggestion, but check out AT&T and T-Mobile, they are the GSM carriers.
Ok, so for at&t, the pricing is here:
http://www.att.com/shop/wireless/pla...paidplans.html