You can't just spend a bunch of money advertising on facebook because your audience won't be geared towards crypto. They will visit the site, get confused what "Bitcoin" is and just close the site.
Hence you need to do your research and find the most efficive method. Many people opened up "crypto casino review sites" and try and get them popular. If they get some organic high search engine google listings then they might get traffic and get high quality revenue that way. However even those are saturated these days.
The most cost-effective promotion method is advertise on biggest sites on markets in crypto industry:
coinmarketcap.com;
coingecko.com,
massari.io, and so on.
Starting with sites that are daily visited by crypto users who will be more familiar with bitcoin, crypto and more easily to accept crypto online casinos.