Well, I spent 32172 diamonds and got the 80 stamina pack 5 times. Hardly worth it, as I don't need anything else other than stamina. I could have gotten better results just buy buying stamina directly and using stamina resets. I can't be bothered to do the math, but I assume it's inside the lame odds they posted or even better.
P. S. I actually bothered to calculate and I should have gotten 3.9 packs with 80 stamina, so it looks like I beat the odds. So even when beating the odds, these chests totally suck, unless you need all the other junk items in those chests.