Not sure what it is these days, but in NSW the regulations on poker machines required a minimum 80% return to the punter.

This was calculated by taking every combination and adding up the payouts and compare that to the cost of playing that number of combinations. For example if you had a 4 reel machine with 20 symbols per reel, then you would have 20 x 20 x 20 x 20 = 160,000 combinations at $1 per single row play, the machine would have to return (on a statistical basis) $128,000 from the $160,000 put in.

Of course, in the real world.......