With residential ISPs... My ISP used to do a (marginally) better job than most. You would pay for speed, with a soft data cap. If you go over the cap by a significant amount (like 10 gigs or more), you'd see a warning on the first and second months. Third month, you're paying per gig of overage, or have the option to upgrade to a faster speed/more data.
I'm not sure if they still do it like that, because I no longer live in a house with 5 other people on Steam. But it's a whole lot better than the barbed penis you get if you are over by 1 kB on your cellphone data.