First, just in case, - how to configure IMAP push: you can find that described in "How do I enable IMAP Push ?" of the FAQ:
http://www.aqua-mail.com/?page_id=227Now about data usage.
What IMAP push does is that it maintains a constant connection to the server (sort of a "heartbeat"), so that connection is ready for the IMAP server to inform Aquamail about a new message(s) right when that arrives.
Maintaining that connection consumes both battery and data.
And I don't think you can change that amount.
If the data is costly to you, you can use the option "Push on Wi-Fi only". If you keep scheduled sync enabled, - you'll still have your message notifications with sync's periodicity (the default is 15 minutes, but you can change that) while on mobile connection.
In any case:
What you should
not do is to apply Greenify or any battery optimizers to Aquamail (push or no push), - otherwise you'll block Aquamail functionality to get new messages automatically (via scheduled sync, via IMAP push, or both).
You should whitelist Aquamail from any battery and data optimizers.