I'll quote it here for you -
WoW uses (not from first hand reports, but from posts on Battle.net) anywhere from 5MB (solo) to 100mb (25-player raid) per hour. You can expect similar data usage I reckon - anywhere from 10-25mb/hr.
That means if you're playing, say, 20 hrs a week on your card (and 10 hours ACTUALLY WORKING, right?) you can estimate make a top-end estimate of about 2gb/month (but likely much lower). Sounds like that's well within your available caps, just keep an eye on usage if you do any media streaming/dl/ul.