It's very difficult to accurately price bandwidth by the GB.
At the wholesale level, bandwidth is sold as a committed throughput rate. Essentially like an "open pipe" of various diameters (to use a semi-bad analogy).
Wholesale bandwidth is currently around $15/Mbit/mo. This means that for $15 you can get a 1Mbs connection to a wholesale Internet provider for 1 month (not really, because they generally want you to buy at least a 10Mbs handoff). If you leave this connection completely idle, you pay $15/mo. If you saturate it 24/7, you pay $15/mo.
Because of the bursty nature of Internet traffic, ISP's will typically resell multiples of their committed bandwidth. You might have a 100Mbps handoff, but could sell 500 or 88Mbps worth of traffic to your customers.
Bandwidth really doesn't get metered down to the MB or GB until you get to the consumer level. This is mostly done to "shape" demand somewhat, and allow for a billing method to charge users who are heavy users (eg: if your customers all tried to actually use that 800Mbps you were selling them and you really only had 100Mbps of available bandwidth, you're going to have to do something about that...). For higher-level datacenter type customers, it's typically a 95th percentile billing method, not a direct $/GB overage charge.
You can do some math and take your operating costs and divide by your bandwidth and figure out your cost/GB, but that's not going to be highly accurate. In many cases you can go from say a 40Mbps connection to an 80Mbps connection with all the exact same equipment, and just pay the upstream for the extra 40Mbps/mo. This would mean the same infrastructure cost is divided over a larger amount of bandwidth, resulting in lower cost/GB (until you need to add another card to your edge router, or add another router altogether...).
I think that part of the problem people are having with coming up with fair and realistic pricing for bandwidth is that unlike gas or water or electricity, you're not really delivering a physical commodity with costs that vary directly by quantity used.