If I remember correctly, you pay for the phone when you buy it and that's it. So just $199 + tax out of pocket at the at&t or apple store (8gb). Then you pay your first month and the activation fee on your first bill (sometimes the first two months if you're at the end of a billing cycle when you buy). The minimum iPhone contract is $70 a month and then the activation fee is $36 I believe, so you pay a total of a little over $300 to get an 8 gb iPhone (if you don't have an at&t contract already) but only $200 and tax at the time of purchase.
Plus, if you never actually intend to keep AT&T's service after you've purchased the phone, then you should factor AT&T's early termination fee into your budget.
It's been stated that would be illegal to unlock your iPhone... Well, the point is moot for the time being because nobody's discovered a hack yet which would allow you do do it.
Even if it were possible, the fact is that doing so almost certainly would not result in any charges being brought against you. If you keep on paying your monthly bills to AT&T in compliance with your contract, they will keep on getting their revenue so they won't care what happens to the phone. And if you sever your ties with AT&T in compliance with their early termination policy, they would no longer have any claim against you. And at least for the next several months, Copyright law in the USA specifically permits circumvention of digital locks for the purpose of making a wireless phone compatible with a different network.
However, you would lose all warranty support for the iPhone and any damage or bricked phones that result from an unsuccessful unlocking attempt, or from any future incompatibilities between the hacked phone and future OS updates, will be totally your own responsibility - Apple will not be obliged to provide any repairs or compensation.
All that being said, it has been shown that a TurboSim method does work for the iPhone 3G. By this method, as far as the iPhone is concerned, it is still locked to AT&T. However, hackers who claim to know about these things say that one of the effects of a TurboSim unlock is that it causes the iPhone to broadcast fraudulent network sign on information over the air (ie. data that doesn't match either the original AT&T network sign on, or the new 3rd party network sign on), which in many countries
is illegal, independently of the act of actually unlocking the phone itself.