Amazon.com hass AT&T Nokia Lumia 520 4" GoPhone Prepaid Windows 8 Smartphone on sale for $39.99 now.
I think this is the cheapest offer for Lumia 520!
That is a great price.
There is no free way to unlock it from AT&T.
But you can use it with AT&T MVNO's like H2O Wireless without any special procedures.
H2O Wireless is much cheaper than AT&T, especially if you are a low-volume caller - I'm paying $10 for 200 minutes that last for 90 days.