Once your phone dies, go to the apple store and they will replace your broken phone with a new one (identical model) for $150 or $200.
Out-of-warranty cost:
iPhone 6 Plus: $329
iPhone 6: $299
iPhone 5s, iPhone 5c, iPhone 5: $269
iPhone 4s: $199
iPhone 4, iPhone 3GS, iPhone 3G, Original iPhone: $149