It varies. I pay 25 UKP a month, about $40 US equivalent, for 8Mb, but that's for a full package with lots of extras. Probably the average figure is around $25 equivalent.
Many combine ASDL with their telephone package, and a couple of those even throw in 2Mb ASDL for free. I would have considered changing, but as yet their customer service has a pretty poor rep - and by far the most important thing to me is that my broadband service is reliable.