iPhone 4 (16gb): $215 and change*
iPhone 4 (32gb): $325 and change*
One time activation fee of $36.
If it's been more than 20 months that you've gotten free/discounted phones from AT&T then you could reduce the activation fee to $18 (when they call it upgrade fee) and still get the phone at the discounted price. Of course that will renew your contract to two more years.
You can always do something called an Early Upgrade where you don't have to pay the full price but also not the super discounted price, you pay something in the middle. But I don't think that's something you're trying to find out.
The cheapest individual plan for iPhone is $64 and change* (after tax). (Plan includes: 450 nationwide minutes, 5000 nights and weekend minutes, Visual Voicemail, 200 mb 3G data, No tethering, No A-List, No text/MMS, rollover minutes expires after a year).
You should visit your nearest Apple store, they will answer all your questions very well.
*NYC sales tax rate.