I like the idea, especially as a caster.
Imagine: Walk into a casino wearing a breastplate, with a sword at your hip and a shield on your back. Looking perfectly the typical "guard captain" with decent armor and weapon.
But in reality, you're a wizard.
With plenty of charm spells Still/Silented.
And you proceed to dominate the place with well placed charms and divination magic.
That being said, there is no reason to use real-world casino rules, as we've had generations to perfect the nature of "the house always wins", whereas it seems this is a new venture in your world. The rules will still be clunky, and the people will still win more often than they do today.
Keep the games simple, it might be difficult for the commoners to play a complex game like poker or Magic: TG (though, they would attract players with ints of 14+ like bards, wizards, etc). It's tough to keep track of all the rules, all the cards, and how they interact.
However, games of chance and dice are beautiful for taking advantage of people with low or average intelligence (read: almost everyone).
Also, not many people will have the funds to come and throw their money away, so standard treasure rates would apply, for the most part. If you want to make it more random than that, give him money per-month instead of per-week, and use your idea of "three d20's with different modifiers", and at the end of the month, roll a d4 and multiply it (on a 1, multiply by .5, on a 2, multiply by 1, on a 3, multiply by 1.5, and on a 4, multiply by 2). This would keep a random "luck" factor in your venture without being too much.