Interesting question- slot machines are actually based on a degree of randomness in each individual play, but the payouts when those random events produce a winning combination are controlled to ensure that only a certain percentage of money played is paid back. Typically in the high 90% range.

Casino slot machines typically have the highest payouts at 95%. This means that for every dollar you put in, the machine is programmed to give back 95% of it at some point. Other gambling locations such as airports, bars, grocery stores, and service stations typically stock machines that have a 50% to 75% payout.