One estimation that you can make is to multiply the house edge, which is usually 0,5% by the total bet amount.
This will be the betting site turnover. After that it depends on the number of employees, hosting, promotions, referral payments, etc.
you are right, i usually do this kind of calculation to estimate how much casinos earn per year or day, actually unless some public casinos like bet365, williamhill, seldom private casinos will public their financial reports to public, that's why we don't see the figure in here, but we can estimate the wagering amount times house edge.