http://www.forbes.com/2001/05/25/0524porn.html
A Forbes article from 2001. Lots of different numbers in there. Everything from $750 million to $14 billion.
And then there's this...
The Business Of Smut: What Is It Worth?
Adult Video: $500 million to $1.8 billion
Internet: $1 billion
Pay-Per-View: $128 million
Magazines: $1 billion
Total: $2.6 billion to $3.9 billion
Sources: Adams Media Research, Forrester Research, Veronis Suhler Communications Industry Report, IVD