How high are the chances of getting a legendary card?

So I got a normal Deathwing in the 3rd card package I opened, and since I usually am like the unluckiest person when it comes to Blizzard games I asked myself what the chances of getting a legendary card are.

Is it compareable to the Dropchances of a certain Epic in let's say WoW (or 'insert Blizzard game' here) or is there even a special percentage per package? How many packages do you normally have to open to get one of these shiny orange cards?

Solution 1:

According to what most people that opened a lot of packs said, there's quite a low chance of getting a legendary.

On average, people get 1 legendary per 20-25 packs. Some are less fortunate than that however.

Source: S1, S2, S3