There are few things more game-dragging than a few player characters deciding to gamble in-game and expecting it to be played out.
So, I dug around the interwebs and found an article on gambling for Cyberpunk 2020 and figured out that this is pretty much what I want – a system that breaks down a full night of gambling into a few die rolls. So here it is adapted to D&D5e:
First, a character must choose how much capital they are willing to risk at the tables in an hour of play. (Standard amounts are usually 10-20 gp, with some high rollers working more in the 100-500 gp range and guards so on operating in the 5 sp range).
Then the character must choose what their target profit margin is. Aiming for greater returns involves greater risks.
Goal – DC
Break Even : Dif 15
50% Profit : Dif 20
100% Profit : Dif 25
250% Profit : Dif 30
500% Profit : Dif 35
These difficulties are for a typical casino in a major city. Small venues and private games are either easier (by 1d6 points) or more difficult (by 1d6 points) because they either fix their games or are making various mistakes that make them easier to break. New venues trying to bring in a fresh crowd will usually also have easier difficulty numbers as they attempt to attract clientele.
Now make a Wisdom check in most cases (sometimes an Intelligence check or rarely a Dexterity check) to determine success or failure. If you are proficient with a gaming set, you can add your proficiency bonus to ability checks you make to play a game with that set. Each type of gaming set requires a separate proficiency. If cheating, a character can add their Sleight of Hand proficiency, but then must also beat the Insight or Perception checks of those playing and watching the game.
If the roll fails, then no money is gained and 10% of the capital is lost for every point the roll was failed by to a maximum of 100% of the capital unless the roll was failed AND a 1 was rolled, in which case the gambler horribly overextended themself and lost 50% more than anticipated.
If the roll succeeds by 10 or more points, treat the gambler as having been gambling at the next higher difficulty level when determining profits.