That number apparently is so difficult to arrive at with any confidence that the XBIZ scientific method consisted of asking 311 people :
"Do you think porn revenues are:
more than $10 billion per year
more than $5 billion per year
more than $1 billion per year
less than $1 billion per year"
I don't see how you could get any more scientific and detailed than that.
Also, they concluded from carefully analyzing the reams of data collected that current gross revenues have decreased "by more than 50% from what was widely recognized as a peak period for adult entertainment in 2005"
So don't ever say that there isn't quality analysis in the adult business.
porn revenues exceed 5 billion world-wide despite free pornography online and rampant piracy
Yet this intrepid lady, who bills herself as "one of the world?s leading anti-pornography feminists" estimates it at $97 billion, explaining thusly:
"I think if you really want to talk about the growth, you?re really looking at a $97 billion dollar a year industry. That?s a rough figure. It?s actually very difficult to come up with any accurate figures of revenue for the porn industry. If you really want to understand how the industry became so big, then you really have to understand the role the internet played in the process of demand and consumption. First of all, the internet made pornography affordable, accessible, and anonymous. Those are the three things that have actually increased consumer demand. "
Pornography and Gender Politics Within Neoliberalism
So two really well titled pieces of thorough, logical research there for you. And to quote Roald, "Cheers!"