In the thread that CuteTali linked to, there is this great piece of information. (I wonder who wrote it )
General Information about special boxes (thank you MiniLaura):
Each time you open a box it costs 24 gems.
When you buy a box, you will get ONE random item from the box. You have a 54% chance of getting a 4th prize, 35% chance of getting a 3rd prize, 10% chance of getting a 2nd prize, and 1% chance of getting the first prize.
If you buy multiple boxes, you may get duplicate items. Your chances do not change if you open subsequent boxes. (So if you open 99 boxes without getting a 1st prize that DOES NOT mean that you WILL get the 1st prize item on the next box. You still have only a 1% chance of getting the 1st prize item.)