How can I reduce the mobile data used by the iPhone Mail app?
There is no way to do this in iOS.

For a workaround you could turn off 3G and turn it on when you need it, just before you open up Mail, that way when you are connected to WiFi you will get push, but manually have to get from 3G.

It's not a great situation: I would just increase your data cap, although I know that is not always easy, hence you asking for this workaround.