Just look at the gem cost of the items which I assume ZOS did not choose arbitrarily. You can probably assume that the odds for each card are around (1/gem cost) without being too far off for the upper tier items.
So if you are looking for a rough estimate of your likely hood of getting an apex mount, I would assume it's around 1 in 400.
PC/NA - Nightfighters, Raid Leader and Officer
Lilith Arujo - DC sorc tank/dps/healer - Dro-m'Athra Destroyer, Gryphon Heart, Grand Warlord
Lilith Tortorici - DC templar trials healer
Publishing odds are an important part of ethical gambling and required for all games in vegas. Slot machines are also calibrated monthly to ensure proper odds.
Its about time that ZOS actually publishes the methodology for their RNG system entirely