I put a sidebar into The Ragged Edge about gambling. These rules might be a bit too dice-rolly for your tastes, but perhaps you can steal something from them:
Gambling is resolved as follows; firstly, the Games Master declares how many rounds the game will go on for (usually three). At each round, each character involved in the game puts forward a stake, then makes a Intrigue check. The results of each character’s check are revealed. Each character can then choose to fold (losing their stake) or keep going to the next round and raise by putting in another stake which must be at least equal to the highest stake in the last round. In each round, the character gets a bonus to his Intrigue check based on his previous result.
Previous Round Modifier
Character got the lowest Intrigue result –4
Character got a below-average result –2
Character got the average result* +0
Character got an above-average result +2
Character got the highest Intrigue result +4
*: Calculate the average by adding up all the results for that round and divide by the number of participants.
The winner is the character who gets the highest Intrigue result in the last round.
Bluffing: A character can Bluff, adding up to +4 for his Intrigue check for the round. Each participant in the game may target a number of other people in the game up to his Wisdom modifier with a Sense Motive check. If the Sense Motive check beats the character's Bluff check, then the bonus added to the Intrigue check using Bluff becomes a penalty instead.
Cheating: A character may cheat, by making a Subterfuge check opposed by a Notice check from everyone else present. All the characters present should make a Notice check, even if no-one elected to make a Subterfuge check. Multiple characters may try to cheat, but the same set of Notice checks are used for all of them. Any character who makes a successful Subterfuge check may add +8 to their Intrigue check.
