How much does it cost to unlock an iphone 4 from sprint for use on at&t?

I have a sprint iphone 4 that i got from my friend for 50 bucks, how much would it cost to get it switched to at&t service?

It is not possible. Sprint is a CDMA carrier, AT&T is GSM.

Unfortunately, you can't unlock your phone to use it with ATT since sprint is a CDMA phone while ATT is a GSM.