I also want to add, what would happen if someone won a large jackpot on day one? That could potentially wipe out your bankroll completely, and if you refuse to payout, you lose you're reputation and any chance of having a successful gambling site.
When launching a casino, the jackpot usually has a small size of lets say 1 BTC for each game and gets bigger with each game made. Thus, the starting capital for the jackpots can be well calculated. As a rule, 0.02 to 0.05% of the bet goes into the jackpot, depending on the casino's prize draw. The longer it is not hit, the bigger it gets.