Apparently you're also supposed to get unique gear, but I never won any after 60-70 attempts. Prizes can be absolutely nothing (a chameleon monster shows up in the prize wheel), ultra vials, money (up to 7500 crowns, that's the max amount I've seen so far). You need a Golden Slime Coin to use the machines, or if you have none, pay 200 crowns. There are 4 machines and some NPCs monsters you can talk to. I have found the Casino the 24/05 at Depth 19 of the Copper Skull gate, in the Wasteworks Slimeway (near the beginning of the level). Check the recent artworks if you don't believe me. As it would seem I'm the only one who has 'officially' discovered the casino so far, I can explain what it's about to all of you.
