Here's a post from a couple of years ago indicating overage of $5/GB.
https://communityforums.rogers.com/t5/Internet/Question-about-maximum-overage-charge/m-p/395107#M454...
If you're concerned you may wish to consider one of the new unlimited plans. I recently switched and my monthly rate actually went down due to the high costs of my grandfathered plan...
I'm not sure I've given you the correct information, I see from the following post that a lower plan was $1.50/GB.
https://communityforums.rogers.com/t5/Internet/Data-Overage-Error-Why-did-it-occur-amp-How-to-quickl...
Best you wait for a mod here to provide the exact number, but my recommendation to consider unlimited still holds. When you login to MyRogers, is there no indication of cost there?