Help with Unlocking iPhone 5

I currently have an iPhone 5 on AT&T. I really want to switch to T-Mobile, and plan to do so after I move away from where I currently live (bad TMO coverage here). I know TMO is running the promotion where they'll pay ETFs if you turn in your device, but I don't want to get an iPhone 5s when I can wait for a 6 around September. So I'm left with keeping my iPhone 5. Is there any way I can unlock my iPhone without paying through the roof...? I'm running iOS 7.1 and my AT&T contract won't run out until October, so there's no chance of them unlocking. Please help a student in need!