I have only played slots for a few hours, but my lifetime average is up ~3x and a few hundred dollars vs. what I put into the machines. So, I think they are set fairly close to break even as that's from a few dozen minor wins vs a single jackpot. Honestly, if every single time someone played for 3+ hours they where always down then fewer people would come back.
That said, they probably set machines in very high traffic areas to terrible odds to catch as many suckers as possible.
That said, they probably set machines in very high traffic areas to terrible odds to catch as many suckers as possible.