I can't help but feel real thick, but how you get to 15GB benefit is not immediately obvious to me. Nor the 29.5GB in your later example.
Let's restate it: Cost = monthly expense. Benefit = average data received.
So a new month begins. On my slower uncapped connection, I pull 1 Gb during this day. At midnight I had 0Gb downloaded. By Noon I had 0.5Gb downloaded, By the next midnight I had 1 Gb downloaded. So on average on the 1st day I had the benefit of 0.5Gb. The next day, another Gb is downloaded, my benefit rises to 1Gb. I continue downloading 1Gb per day. By the end of 30 days, I have downloaded 30Gb, and my benefit has risen to 15Gb.
On a graph, see y=x, a line starting at the origin, (0,0) at a fixed gradient, ending at (30,30). The benefit is the average area under the curve, or more formally 1/30th of the intergal of the curve y=x over the interval x=0 to 30. This equals 1/2 * (30^2)/30 = 15.
In the second case, my super fast capped connection pulls 30GB on day one. At midnight I had 0Gb downloaded. By Noon I had 15Gb downloaded, By the next midnight I had 30 Gb downloaded and I'm capped. So on average on the 1st day I had the benefit of 15Gb. Since I'm capped I can't download any more, so for each subsequent day of the month I have the benefit of the 30gb I downloaded on day one. The benefit over 30 days = 29.5Gb.
On a graph, see y=30x, a line starting at the origin, (0,0) at a fixed gradient, ending at (1,30). For x>1, y=30, a flat line. The benefit is the average area under the curve, or more formally 1/30th of the intergal of the curve y=30x over the interval x=0 to 1 and the integral of y=30 over the interval x=1 to 30. This equals [30/2 * (1^2) + 30 * 30 - 30 * 1 ]/30 = 29.5GB
I hope this explains it better, rather than making it even more confusing...
The best way to look at it is to imagine data recieved on the y axis of a graph, and days of the month on the x axis. Benefit is the average amount of data received so far, averaged over the course of the month.