The Economist has an interesting discussion of several academic papers addressing aspects of the question. Shane Greenstein of Northwestern University and Ryan McDevitt of the University of Rochester calculated the consumer surplus generated by the spread of broadband access (which ought to include the surplus generated by internet services, since that is why consumers pay for broadband). They did so by constructing a demand curve. Say that in 1999 a person pays $20 a month for internet access. By 2006 the spread of broadband has lowered the real price to $17.