I don't believe there is any way how you would be able to find out that information since it is confidential information. You cannot call Visa to find out how much your neighbors credit card balance is....same thing with trying to find out how much a webmaster makes. However, if you check the sites rank on
http://www.alexa.com, that will tell you how much traffic the site is getting but you will not be able to find out how much $$$ the site generates through alexa.
The lower the alexa rank, the more traffic the site is getting...and in most cases, the more traffic a paysite generates, the more $$$ they are generating. But yeah, there is no way of actually finding out the amount of $$$ the site makes unless someone at the processing company gives out this information and if they were to get caught doing this, there would be a very strong possibility of them losing their job.
My advice is to not be so concerned with what another webmaster is making but be concerned with what you are making.