While saling on the Miracle a couple of years ago, my wife, won over three hundred dollars on the first night playing the slots. I don't think we put more that $2.00 in the machine before it hit, which more than paid for our on board expenses.
Just remember, the casino, like photos, drinks, and all the other shops on board are there to produce revenue for the cruise line. If you treat the casino more as "entertainment" than a place to "strike it rich" you're better off. After all, "you need to know when to hold them, know when to fold them, know when to walk away know when to run".