If you're lucky enough to qualify for one of the "early upgrades" that get you out of the usual 18-month AT&T commitment period, then you pay just $199 for 16GB of joy and $299 for the 32GB model.
If your upgrade date is January 1, 2011 or after, early update pricing for the iPhone 4 will be US$399 for the 16GB model or $499 for the 32GB model.
Should you want a "no-commitment" contract, meaning that there's no early termination fee for dropping the phone in less than two years, the price sneaks up another $200 to $599 for the 16GB model and $699 for the 32GB unit.
You can check your upgrade options via AT&T's myWireless account administration tool, or by dialing *639# (*NEW#) from your iPhone.
As with any major purchase, you should consider these pricing factors before you jump into purchasing a new iPhone. Remember, there will probably be another new model coming out in June of 2011, too.