I'll try to answer this.
Nowhere else in the networking industry do you pay for access to pipes like this by the gigabyte. So the question, unfortunately, starts with the presumption that you could put a price on a gigabyte.
If you had to, you would make it in the few cents, easily less than 10ยข, range. Unfortunately, it's just the wrong way to show the cost of a gigabyte. As for how people price a gigabyte, that's market forces.