Since you have given an example of how much it should cost for a gambling site to spend on advertisement, then maybe it's better to give an example too of what gambling sites literally end up advertising on all those forums and social media platforms on the list. Maybe I'm wrong here but I don't think there's a gambling site that puts that kind of advertisement on all platforms as part of their advertisement.
Let's also add the fact that famous gambling platforms nowadays don't need too much exposure on various platforms as they are already popular. With just a partnership or exposure only on a most viewed platform, that's already enough form of advertisement as what these popular gambling sites are doing now is just to maintained the popularity and keeping it that way.
The advertising was the key factor for the gambling site to run for the longer period.When the gambler browse about the gambling site,if you want to view your gambling site as first.The maximum amount should be spend by you on the advertising and all social media pump.Which is commonly called as the shilling or digital marketing,the social media advertising plays huge role to reach maximum number of gamblers.Once the gambling site get good reach,they should not stop the marketing and it may reduce their popularity.