There are different angles we can look at to answer this question. Let me start with absolute numbers. As an example we take the annual report of UK’s number one sports betting provider Bet365. The fiscal year 2018/2019 ended up with an incredible £2.98 billion in revenues which is an increase of 10% compared to the previous period. The after-tax profit gained 16% to £681.7m.
If we now look at the £64.5 billion (up 22.7%) wagered by the customers and compare it to the revenues, we see that the bookmaker keeps 4.62% of the turnover. And now we are coming to the point. Looking at the payout of the bookmaker, we see the numbers of the fixtures pretty much copying the annual revenues. We do NOT know how much of the revenues goes to online casino and how much to the sportsbook, but it seems that Bet365 is very good at what they are doing.
I personally always look at the odds of Bet365 before betting for real. When searching for value, I have 2 rules.
Rule 1: The higher the difference between the highest odds offered by any bookie and the odds of Bet365, the higher value.
Rule 2: If odds comparison shows Bet365 giving the highest odds, consider this as a hint to bet on the opposite outcome.