That 250 gigabytes-per-month works out to about one megabit-per-second, which costs $8 in New York. So your American ISP, who has been spending $0.40 per month to buy the bandwidth they’ve been selling to you for $30, wants to cap their maximum backbone cost per-subscriber at $8.
I upgraded to “Extreme 50″ DOCSIS3 recently. Essentially I’m paying $100 for the same $8 worth of backbone cost. 250GB is definitely very tight, 500GB would be more reasonable.