Is there a way to see how much gambling sites makes?
Well not really if the gambling site doesn't list their stats but most of the bitcoin casinos these days do offer you insight to their stats.
Most crowdfunded bankroll casinos will be able to get you their stats such as profit and investor profit, etc. etc. however most casinos that only have their admin and co-admin as founders don't allow you to look inside their stats.
You could email them, i haven't tried it before i don't think if it works or not.
You could look on DIcesites as well, that's a great site for casino stats.