Maths is wonderful, right? Unfortunately (fortunately for you!) most of the population have fail maths abilities. I call this a "scam" because you can never lose money, only ever win it. Perfectly legal and the players will appear to be betting against each other, but you're actually raking in a fortune.
This is (imo) better than systems such as:
1-60 lose money
60-99 2x money
99-100 3x money
If you get some guy betting large amounts (and then fluking a 100) you could well have to pay out quite a lot. And if you keep the money then Blizzard might action you for scamming.
In any case, here's my version (this won't cover HOW to advertise and use macros to keep things moving, read another guide for that)
A good advertising line is "6 player casino, win up to 4x your money! Bet against your friends!"
Ask the 6 people interested to each pick a different number between 1-6 (basically they just each need a different number) and to decide an amount they want to bet (each person bets the same amount). You then (use a macro)
/random 6
/random 6
/random 6
If their number comes up ONCE, they get 2x their money
If their number comes up TWICE, they get 3x their money
If their number comes up THREE TIMES, they get 4x their money
Explain that you pay them based on the money you get from the others and out of your own pocket (don't worry, that's a lie, you'll never have to pay anything. It's just to make them feel more comfortable).
Below I outline the three possibilities and potential earnings for each. These numbers are based on a bet of 50g with Player 1 picking the number 1, Player 2 picking the number 2, etc.
Possibility One
All three numbers different. Probability of this happening is 56% (roughly) so it should happen about half the time.
Amount traded to you before rolls: 300g
What you do: (if numbers rolled are 1,3,5)
Player 1: Give 100g
Player 2: Nothing
Player 3: Give 100g
Player 4: Nothing
Player 5: Give 100g
Player 6: Nothing
Your profit: Nothing
Possibility Two
Two numbers the same, one different.
Amount traded to you before rolls: 300g
What you do: (if numbers rolled are 2,2,4)
Player 1: Nothing
Player 2: Give 150g
Player 3: Nothing
Player 4: Give 100g
Player 5: Nothing
Player 6: Nothing
Your profit: 50g
Possibility Three
All three numbers the same
Amount traded to you before rolls: 300g
What you do: (if numbers rolled are 3,3,3)
Player 1: Nothing
Player 2: Nothing
Player 3: Give 200g
Player 4: Nothing
Player 5: Nothing
Player 6: Nothing
Your profit: 100g
So there you have it. You can NEVER lose money with this, only make it. You make money slightly less than half the time. And there's nothing anyone can do. I just ran my ingame macro 10 times and I got:
3 different: 6
2 same, 1 different: 4
3 same: 0
If people were betting with 50g stakes, I'd have made 200g.