Simply have $800,000 of coin in per day...
You realize that it does NOT say that there's a single $750,000 jackpot, right? Over $750,000 is the total paid out. And keep in mind, that counts the amount paid as credits that are re-bet and lost.
Do the math - Stop and think how much a typical machine takes in.
A $1 slot has a max credit of, what?, $3 per spin? At an average of only 10 spins per minute (it's probably faster), that's $30 per minute or $1,800 per hour.
You only need 35 machines, at 12 hours per day, to cover $750,000. There's probably way more than 35 of these machines averaging more than 12 hours a day....