If you go a little over the 100MB we won't pounce on you. And 200MB is more than you are likely to ever need. You also get a huge traffic allowance with the Gold Accounts. Of course its no good to buy into something that doesn't work. From my point of view, its not worth it to sell something that doesn't work, or breaks down.
Economies of scale is a part of it, and low overheads.
We are part of a large buying pool. The fact is though, there are cheaper offers out there - but if you read the fine print they give you less traffic and not the kind of features you'll get with us. We're comfortable with our margins here. We buy in bulk and pass the savings on. But we're hoping to generate consulting work for ourselves through contacting people in this business - this helps as well.