How much does it cost to unlock an iphone 3GS?

At&t iPhone 3GS 16GB

No 3gs on atnt. Iphone 4 costs 479.99

Wait do you mean your locked out. Then if so, all you gotta do is factory reset it.

I think Burn answered as if you were thinking of buying a 3GS. He/she is right, AT&T no longer sells the 3GS. But if you already own one, there are two paths to unlock:
1. Go to an AT&T store and ask if the phone is covered by contract. Whoever originally purchased the phone may have finished the 2 year contract. If no contract, then simply ask AT&T to unlock it. They have a new policy to unlock. Then activate the phone wherever you want
2. Go to a mall kiosk or google 'Apple Unlock phone' and several stores in your area will appear. Pick one. The cost varies from $30 to $50. I do not recommend this path.
It is first very important to learn from AT&T if the phone is on their list of stolen phones. If you take path #2 and pay to unlock, then try to activate, the phone will be useless - because it is on a national list of stolen or lost phones. You would be out the money paid to unlock!
Please save yourself time and money by visiting AT&T first to unlock.

You could check with AT&T themselves & see as they unlock off contract iPhones or you could go to http://chronicunlocks.com, where they do it for $20.

$19.99

go here: http://gsmunlocklabs.com/unlock-iphone/

The best place that i have used is iphoneunlockedcheap.com great service and fast, it was like $15