Right after the announcement of Happy Aquarium's new Mystery Treasure Chests, I bought myself four of the things for 100 Coins each and waited a day for 'em to open. The results can be seen in the image right below.
The 4 items, which were 2 "'Beware' Signs" and 2 "Octopus", were less than stellar, but they were easily resold to net me a profit of 72 Coins. So I tried buying 16 Mystery Treasure Chests, and waited another day. The results indicated that there was a pattern forming.
My Mystery Treasure Chest Results:
4 x Octopus; costs 145 Coins at the store; resells for 116 Coins
4 x "Beware" Signs; costs 150 Coins at the store; resells for 120 Coins
4 x Glacier Point; costs 153 Coins at the store; resells for 122 Coins
4 x Bonsai; costs 154 Coins at the store; resells for 123 Coins
Since I sold back everything, I had spent 1,600 Coins, but got back 1,924 Coins. If you've got the patience to wait, the Chests could be a good monetary investment, because it's like it's guaranteed that you'll more than break even. But the whole point of the Chests is to get a random assortment of items, and the item selection was oddly consistent.
My Verdict: For 100 Coins, you can't expect too much. You'll get your money's worth and then some, but that's not the point of these grab-bag deals. They seriously need to grind some Gremlins into their randomizer to liven up the results. At least the Chests look cool.