Once per day, the player can win a random prize from the raffle.
Unlike other shops, the Raffle Shop's merchandise pool doesn't
increase as it levels. Instead, the player's chance of getting a top
prize is increased.
This is very randomized and you can't really stick a percentage on it.What does happen is that the higher your raffle's shop rank is, the more chance there is to actually get the Master ball.
This isn't a really reliable source to get the Master ball. You get 2 master balls in black/white 2. One when you get from Profesor Juniper when you just enter Mistralton City and the Other you get from Colress.
If you need the master ball to catch legendary easy then I can tell you that there are tricks of catching them easily
The answers on this link give handy technics for catching Legendary Pokémon
Hope I helped in a way