There is no rule of thumb when wagering in pool. Contrary to what a lot of people think, wagering on billiards is not illegal in America. The reason for this is because you are wagering on skill, not on a game of chance. Most 9-ball players like to wager on sets. They set a target, like first one to win 5 games gets the money. In pool lingo, you would challenge someone in a race to 5 for, say, $20. There are other scenarios where better players entice poorer players to wager. This is called giving up weight, or handicapping an opponent. For example in 9-ball, a better player might give the opposing player an extra money ball, like the 8-ball. If the opposing player legally pockets the 8-ball or the 9-ball they win the game. Another handicap is giving a two game headstart. Another is giving 2-1 on the money. Another wagering game in 9-ball is a "ring game". There are usually 3 players involved in a "ring game". Wagering is usually done on the 5-ball and the 9-ball. For example, three players decide on $5 on the 5-ball and $10 on the 9-ball. When one of the players legally pockets the 5-ball, the other two pay the player $5 each. Same with the 9-ball. A ring game is the easiest way to "hustle" someone, because two of the players are usually working together undetected. Other than a ring game there are usually no "per ball" wagers. Wagering in 8-ball is usually done on a "per game" basis,but not exclusively. A fun game to wager on is "three-ball". For example, you get 7 players together. Each one tosses a dollar into a pot. Players take turns pocketing a rack of three balls. Whoever pockets the three ball in the fewest shots wins the pot. If two shooters tie everyone throws in another dollar and everyone shoots again. It's called "one tie, all tie". The more players, the better chance someone will win a larger pot. Sometimes it comes down to the first shooter getting a 3, the next five players getting 4's and 5's, and the last player tying with another 3, or winning with a 2. The main rule of thumb is: If you think you are being "hustled" you probably are. If you don't think you are being "hustled" you probably still are.