Basically you got two options.
pre-paid (payasyougo) and post-paid
The pre-paid basically sucks big time, especially since in America you get charged for incoming and outgoing calls & txts.
If you go with post paid, then the default policy is to put you into a 2 year contract (person gets commission) but a quick talk to the manager explaining how you are bringing your own phone and they won't do that to you. As for the cost, it would be the same whether you are in a contract (bought a phone subsidized) or not.