As an individual? T-Mobile is the cheapest. You won't get 3G speeds tho and will need to unlock the phone, which is possible if it's never been on FW 4.1.
AT&T can get pricey but you'll have fast 3G speeds and no need to worry about unlocking.
T-Mobile Individual plan you'll probably be looking at around $40-$60 depending on if you want texting and if you implement the 9.99 web2go trick. AT&T you're looking at $70-$90 depending on if you get texting or the 200MB or 2GB data plan.
In most states, $70/month after taxes is typical for at&t on the cheapest voice + iPhone data plan. That is without FAN discounts.
I use T-Mobile's EM+ 500 minute plan with the 9.99 'trick' (which really isn't a trick as they only enforce plans on their branded phones as does at&t). I pay around $45/month. Give or take a couple bucks for taxes depending on state.
You might also look into any regional GSM carriers, if any. They usually offer cheaper rates if you don't need nationwide coverage.
If you like texting, but don't need data, you can do GoPhone and get unlimited talk+text for $60/month (tax inclusive).