How much would it cost to get an iPhone 4 battery replacement?

I bought a second hand iPhone 4, of which the warranty has expired. The battery is old and needing replacing, how much would this cost?

You can't exactly replace the iP4 battery just like that, you'd have to literally smash the back cover off & then replace the battery. Not ideal. Best thing would be to buy a new one, but that'd be a big expense.

You can do it by yourself:
http://www.ifixit.com/.../IF182-000. Or you py 39$ http://www.missionrepair.com/Mission_Repair_iPhone_4_Battery_Repair_Service_p/222-4108.htm