http://wiki.spiralknights.com/Stranger_Prize_Box_Promotion_November_2015

We're looking at two entirely new costumes- i.e. more than just a hexidecimal tweaking as the common get, with 2% for the aura and a 23% of getting a snipe perch. This is one of those boxes where missing the moon lands you among the stars, which is definitely something that makes me feel good about buying into it... so if you've been waiting to show your support for the game (and don't want to flat out buy wealth), now would be an excellent time to invest a few bucks.

Know your odds:

Getting zero accessories (per box): 75%
5 boxes: 24%
14 boxes: 2%

Getting at least one accessory (per box): 25%
5 boxes: 76%
14 boxes: 98%

Getting an aura (per box): 2%
5 boxes: ~10%
14 boxes: 25%

Divine/Prismatic/Volcanic/Shadow costume -or- an accessory (per box): 52%
5 boxes: 97%
14 boxes: 99%

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Which is to say that this is HIGHLY unlikely to bite you, and that's what I like to see. I'll post my results later when I get home.