How much does it cost for a new IPhone 4 battery?

My IPhone 4 was working perfectly fine a few weeks ago, and then the next day, it stopped charging. My IPhone had eventually died, and now that it had ran completely down, it won't charge, and it won't even show that thing when you try to open your phone and it tells you you need connect it to the charger because your phone is dead. I think my battery just burned out, I've had my phone since Christmas 2011, and it happened in the end of March. How much does it cost to get my battery replaced?