That is a bit hard to answer. I don't think there is a fixed number that is going to cover all your expenses and possible withdrawals, because of so many variables in the gambling industry. First of all, are you planning to work all alone on the casino or do you need employees? It can be quite expensive if you hire support staff, IT specialist that run the website and casino for you, and a security system to make sure there are no leaks and your money is save. The more people that work for you the higher your monthly running cost and your 100k USD could spent quickly. Another factor that can be quite expensive is advertising cost. It depends on how fast you want to grow your casino, to attract a large number of gamblers you need to run some ads to promote your casino. All this will be initial cost that only after a few month will be recovered by money made from your casino. And the last big factor is your bankroll to cover potential outflows. It could happen that a whale plays at your casino, becomes lucky and wins 50k USD. That would ruin your bankroll and needs to be addressed quickly. It's probably better to make sure that you other monthly income that you could rely upon to cover potential outflows.