If Geary meant 5% of the gross bets, then I agree that the figure was totally unrealistic,and was included as dramatic license by FFC. When you consider that games like Craps,Blackjack and Baccarat have a house edge,or "grind" of 5% or less,there is no way to pay out that kind of "squeeze" money.
If the Casino's edge on Craps is 1% of the gross wagers,they can't pay out 5%.
On games like Slots(the biggest revenue generators by percentage),you're looking at 10-12% house edge on gross wagers.

It would make a little more sense if Geary demanded 5% of the gross profits,not the gross wagers,but it would still be a boatload of cash to hide.
When Trump opened his first Atlantic City casino,his nut was a million a day. This was back in the late 70's so even then,
30 million a month was only break even.

I can't see any reason for Geary's demands,except to make us feel less sympathetic when he got set up with the dead whore.