I have centurylink and never ran into the problem of the data limit.
They do have a limit in place of 300 GB and I think if you do happen to go over that limit then it's something like $10.00 per every 50 GB after that.
Not a bad deal and I've never gone over even streaming youtube on roku for half the day.
When I was using verizon mobile data I was getting charged $15.00 for every 1 GB over.
So if I happen to go over my 300 GB data then $10.00 would be a great deal.