The data usage won't actually go up. Autosyncing an IMAP account is done by the server - when there's new email the server sends it to you. Manual syncing means that your phone sends the Gmail server a request for new email, and gets it if there is any. Every time you manually sync you're wasting data. Not much, almost never enough to affect whether you go over your allowance or not, but a few bytes. Unless you check your email every 2 minutes - that can eat a bit of data. (A few hundred bytes per request times 730 times 30 days/month adds up a little.)
The setting is in Settings/General/Accounts. Go to the account, press the email address and turn on or off whatever you want autosynced or not.