In about 1 in 750 packs of first run ZEN packs, one of the following cards can appear in place of a land.
The more powerful and expensive cards appear less frequently than the less expensive of the cards listed.
The following is a (now alphabetized) listing of all the cards that have been reported to have been opened from packs thus far. If the card is on the list already, don't post here. This thread is not keeping a tally of opened cards, merely documenting the different cards in this promotion.
First Run packs are all packs of Zendikar currently available for purchase. Second run is shipped in November. You can tell what run your box is by checking the bar code. If it has an 'A' at the end, it is First Run.
The Power 9 (some from unlimited, some supposedly from beta, some signed.)
The title says 78/?, but there are still only 76 cards on the list. I think you forgot to actually add those last two.
(The two you deleted being Moat and Braingeyser, in case you'd forgotten; whether you meant to add them or not, and whether they're reliable or not I obviously can't say, but given your "78/?" I think I should note this.)
I'm just back from GP Paris and that sample of boosters opened will probably provide the best estimate you can get for the probability of opening hidden treasures, because there was a huge amount of boosters opened and they were from consecutive displays/cases. I played in one half of the tournament with about a 1,000 players aka 6,000 boosters opened. Three treasures were pulled.
In the PTQ on the next day with close to 400 people / 2,400 boosters we had another treasure (Mox Sapphire).
The sample is so big, that it is extremely unlikely that the chance of opening treasures is 1:750. GP Paris obviously suggests it is much closer to 1:2,000.
If you are interested I can probably find out about the treasures in the other half of the GP, too. Btw I have seen another 2000 boosters opened at other events and no treasures. Makes four treasures in about 10,000 boosters for me.
An approximation can only be 100% accurate if you account for every booster and even then it could still be slightly off so what you say about GP Paris' Treasure/Booster ratio is irrelevant