I have the 2 GB plan and I'm about to go over if it only will cost $10 which is what the representative told me on the phone. I started this conversation through email though, where I was told I would be credited $15 this once because I was unaware of the fact that I actually wasn't allowed 2 GBs of data this cycle (my first billing cycle with this new phone).
Why wouldn't they credit just the $10? The phone rep assured me that I'd only be charged $10 if I went over 2 GBs. I asked if I'd get charged per MB or KB thereafter and they said no. That sounds to good to be true though and the fact that they credited $15 makes me wonder if there are hidden charges.