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