Greetings Aionwen,
When doing your probability calculation, You place 10 tiles with minimal risk and maximum cost, and calculate the odds based on a 2/3 chance. This is just one of the total probabilities. The other probabilities must also be calculated and averaged. So, your calculation method is wrong, probability calculation is not done this way.
Additionally there is other reward; the main goal here is to find 100xToken. You should add that your calculation too.
You might even get 100x tokens on your first try by placing a single tile. Or, if you place 5x tiles on that symbol, you'll win 500x tokens.There are other possibilities.
That's why this isn't a direct purchase event, but a game of Luck. So even at the first 1/15 rate, your chance of finding a token the first time is only equal to your chance of finding it the 15th time.
(Although the price varies from mount to mount) You can sell 100xToken in the range of 6-7KBC. This amount will give you the cost of approximately 500 tiles. And don't forget the additional tokens you find while searching for it.
With all these possibilities, the costs per piece will decrease significantly when evaluated based on the total number of games. That's exactly why, as you said, the AH prices of these mounts are much lower than your cost calculations. Otherwise, no one would sell them at half their cost according to your calculating.