I don't know what's a minimum figure but I would easily keep 500 - 1000 btc aside and I know it's a very high amount but I have seen big whales play that high and once they come they need quick payout. Secondly you'll need a good budget for security, upgrades and most of all marketing your site, I would advise you to see how the pros do it before you jump in.
This isn't 2015 anymore where BTC was $200 and everybody had 500-1000 BTC in their cold storage. At $7500 you are talking about $3.75 million up to $7.500 million. Even if he had that much money, he wouldn't risk it all on a casino.
Like everybody said here before. You need to find ways to manage risk like limit the bet size. The way greed works its difficult. Because many casinos want people to risk more since they have a greater chance of making a profit with the house edge; but it can also clean them out.
You want a perfect example, look at what happened earlier this week with the OKEx $500million dollar liquidation long position. The guy was long about 50K BTC and the position was liquidated and there wasn't enough buyers and its still sitting in the orderbooks and every profitable short trade might have to take a loss. This is a perfect example of how badly OKEx has managed risk.
