The average chance of Small Chests from the Market dropping shards:

5 shards - 1,35%
7 shards - 54,40%
9 shards - 13,85%
12 shards - 23,50%
15 shards - 1,20%
20 shards - 2,20%
25 shards - 0,75%
30 shards - 1,10%
45 shards - 0,75%
50 shards - 0,90%

Please note: The figures shown here were taken from recent statistics and have been averaged out.