You have to switch to a small business account instead of a residential account with comcast. Then you get genuinely unlimited use, no bandwidth cap. That is the only way to get past their 250GB cap. It's absurd that they won't sell you more bandwidth on any residential plan at any price, and that they kick people off the service rather than charging overages.
You do not need to have a business location to do this, or actually have a business. They will sell you small business cable internet at your residential apartment.