Warning: This is a small sample size, so numbers at the bottom aren't exact. A more accurate test would consist of at least 1,000 Keys used, but for now this does give an idea of what to expect.
Spoiler: Test 1Show
So first up I had Tarsan farm up some coins, and not knowing what to expect I didn't want to drop a lot of coins only to come out way behind where I started. The Timeless item, costing 7,500 from the vendor was a nice surprise at the time, and easily paid for the whole test by itself.
Spoiler: Test 2Show
Next up I sent Herc around the island. As the previous test was small, I wanted to repeat it and see if I got similar results. Roughly he earned the same amount of coins as Tarsan, but the lack of anything beyond coins made the test set him back about 2,000 coins.
Spoiler: Test 3Show
With the demo test out of the way, it was time to get some real results. After a nice chunk of Rare farming, once again Herc went forth to spend his hard earned coins. BAM right off the back we got not only enough Timeless pieces to pay off the whole test, but then a bonus piece as icing. Coin turnout was still close to what was seen in the first test, so after 50 keys the coin flow is already close to the avg. coins per chest listed at the bottom.
Spoiler: Test 4Show
After a few days break from the isle it was time to return again and continue the tests. At this point I was pretty certain that I was certain the listed chance at the Pet and a Burden of Eternity on wowhead was still wrong so I would have to keep going to get better numbers. Low and behold, a Burden drops on Chest 2, and I dropped my jaw. Being that it cost 50,000 to buy it, it not only paid off this test I was doing, but also the cost of all the test I would be conducting here.
Spoiler: Test 5Show
This test was done just not long after the previous test, with the gains Tarsan got on the last test, and with the odd coincidence all the big rares spawned one after another, he got the 15,000 coins to finish off the testing very quickly. As the between the coins coming in and the early Timeless piece, the test was already a profit, but then ANOTHER Burden drop?! My opinion on the cost of the gambling area was set. Unless Blizz changes drop rates again, then more or less its a better cost of your coins to spend them gambling then saving and buying Timeless pieces/Burdens from the vendors.
- Avg. Coins per Chest = 295
- % chance for a random Timless Piece = 7%
- % chance for a Burden of Eternity = 2%
- % chance at Bonkers = < 1%
For the cost of 20,500 coins (spent 50,000 coins, and earned back 29,500), I received 2 Burdens, and 7 Timeless Pieces. A 152,500 coin value. IMHO, the gambling is the way to go if your looking for Burdens or Epics