The casino unlocks after you get the wand Mornstar in Chapter 4. It is accessible via the door in the Tombstone Trail which was locked during your first visit. These highest level prizes unlock after you finished the main story. All in all, they cost 75,000 chips.
Given that you can buy that amount of chips for 375,000 guilders and you need 500,000 anyway for
, the easiest way is to save up the money in post-story play and buy the chips instead of trying to gamble for them.