Counting the cost of "Are penny slots worth it?" starts with the stake per spin. Penny slots and progressive jackpots sit at opposite ends of the same machines. One promises a small stake, the other a huge prize, and both are less simple than they look. What a penny slot really costs per spin, how a jackpot meter grows and what the odds of hitting it are decide whether either is worth playing, and neither rewards a strategy the way their fans expect.
Progressive jackpots work by taking a small share of every stake and adding it to a prize that keeps growing until someone wins it. The jackpot meter on the screen shows that running total, and on networked games it rises with stakes from many casinos at once. A mystery progressive jackpot is paid at a random moment before the meter reaches a hidden limit rather than on a symbol combination, so any spin can trigger it.
Penny slots work on the smallest coin value a machine accepts, but almost no one plays a single penny. Each spin multiplies that value by the number of lines and by a bet level, so a penny slot with fifty lines costs fifty times its coin value or more on every spin. That is why penny slots are so expensive in practice: the headline value is small while the real stake, and the speed of play, add up quickly. How much a session costs follows from that per spin figure.
They show meter levels and history, which says something about when a mystery jackpot is near its limit, but nothing about ordinary progressives.
On some machines only the maximum stake qualifies, or it raises the chance; the rule is in the game's help screen.
A jackpot paid at a random moment before the meter reaches a hidden limit, rather than on a symbol combination, so any spin can trigger it.
A small share of every stake on the linked machines feeds the meter until someone wins it, then it resets to a seed amount.