If you have a friend that has an AT&T account, you could try getting them to ask AT&T to unlock it for you. If not, then there are services that can remotely unlock your iPhone but they are pretty expensive IMO.
iPhones can also be unlocked by Remote unlocking. It is a safe and simple method!! I unlocked my iPhone 4 at , by this method. The price is little higher than what you asked for, but they offered good unlocking service. Check it out!!!