Unfortunately the cost saving from multicast assumes a traditional IP network. In this respect DSL still looks like dial-up did, with every subscriber at the end of a PPP connection. An ISP gets a whole pile of point-to-point links from a nearby POP. The physical infrastructure is largely hidden from ISPs, so there is no way to save money on the expensive long-haul. If you've got data in London, and it needs to go to 600 subscribers in Cardiff, you have to send and pay for 600 copies of the data.