It uses GBs of data...kidding.
At some point, I figured it out how much data a typical 160 character iMessage is. Let's just say you can send/receive 10,000 iMessages using only 20MB of data (and that's being generous).
If you just allotted 100MB of your data a month for iMessage, you could send/receive 50,000 iMessages. If you send/receive more than that, then I think you're insane.