Comcast advertises very clearly that unlimited is an additional $50/m: https://dataplan.xfinity.com/unlimited/
It'll be paid. Most of their customers don't have any other choice but to pay it. (Otherwise, most of them already wouldn't be their customers, after all.)
Please tell me, how do I avoid paying overage fees? I have zero moral qualms about defrauding Comcast to the maximum possible extent.