Payment for the phone has nothing to do with the battery usage of the phone phone with data capacity or capability since its supposed to constantly fetch data for you so that you can always constantly get a update on your emails your messages and any other notifications that you have on. There are proper measures to take when using a data enabled device, basically manage your data by allowing it to fetch certain things automatically that you want or don't want to. The idea of the smartphone is ultimately have everything at your fingers or at your fingertips automatically, which is the case for data fetching.
Phone hardware a program to do certain things such as drain battery based on the usage. If your brightness is at maximum obviously your battery will drain faster as the brightness of the screen demands more from the battery. If wifi is on 24/7 it'll constantly look for wifi connections when you're not connected to one. Like cell signals, if the strength is weak, it'll push to get a stronger signal, which uses more battery
Disabling "push mail" to save battery life is a MYTH. If you use iMessage, ANY notification based services for an app or iCloud for syncing then turning off "push mail" is USELESS and actually will cause more battery drain if you switch to "pull mail".
You would have to turn off ALL of the systems I listed plus a few not listed to save even 2% battery life in a 18 hour usage period.
Please STOP spreading this useless MYTH. It is counter productive to actually helping someone.