Here’s a quick abstraction I threw together for my players, who are enjoying some down time in Port Zangerios whilst some PCs are training and/or researching spells:
Gambling: For each day spent engaging in games of chance, choose a buy-in amount of 1–250 gp, and roll a d20 on the table below. (Anyone possessing a luckstone* gains a +1 bonus to the roll.)
d20 Roll Result
natural 1 lose 100% of buy-in
< 9 lose 50% of buy-in
9–12 break even
> 12 win 50% of buy-in
natural 20 win 100% of buy-in
*Such as the PC purloiner, in our case