This is according to my own experience. I have 257 arena wins and I like to keep stats. I searched previously but couldn't find an official source for this.
1st, 10th, 20th, 30th pick in Arena are always rare or higher: I call this Rare Turn, the others are normal turns.
At any normal turn, you can get one of these sets:
- 3 free
- 2 free 1 common
- 1 free 2 common
- (possibly 3 common as well didn't pay attention, can't verify this)
- 1 Rare turn ( % not sure I would guess about 5% - 10%)
In a normal turn roughly you have a 60% chance of getting a free and 40% chance of getting a common card.
At any rare turn, you can get one of these sets:
- 3 rare
- 3 epic
- 3 legendary
I got a legendary about every 10 games and since we have 4 rare+ picks I assume the chance of getting a legendary in a single rare turn is about 2%
I got an epic about every game so I assume the percentage for that is about 20%
- Rare+ : 4 times
- Epic : 20%
- Legendary : 2%
I don't have a source for this its all according to my personal experience. These numbers are most likely wrong, but should give the general idea. More data is needed for better estimates.
In a normal arena deck you will have something like this:
- 15 Free
- 10 Common
- 4 Rare
- 1 Epic/Legendary
Of course if you are lucky you could get 4 legendaries, 10 rares, etc..