At /my scale/ I can buy bandwidth at around $1 per megabit/sec. 1Mbps is, uh, around 320 gigabytes of data. Even at half utilization that's less than a penny per gigabyte, and any but the smallest of local ISPs are going to dwarf my bandwidth usage. This is the sort of market where cost scales down quite a lot as you scale up, so I would bet quite a lot of money that comcast pays less than I do.
sure, if I wanted to double my usage, I'd have to buy some new network equipment, and spend some network admin time, but generally speaking the cost of your network equipment (at least at my scale) is fairly small compared to the cost of buying the transit in the first place. It certainly doesn't double the cost of bandwidth; and even if it did, bandwidth would still be cheaper than the local loop to your customer's home.
So yeah, uh, your average dsl is what five megabits/sec on a really good day? are you seriously trying to tell me that you can buy a copper loop for less than five bucks a month?
(Actually, I've been in the industry all my life and I've never been involved in buying copper loops; I guess it's possible they are that cheap; If they are, and I find out (oy, the costs of almost everything in this industry are guarded like state secrets. It's such a pain in the ass.) I'll start a DSL ISP. but I'd be very surprised if that were the case. )