Hmmm, so EACH of the 11 cards pulled has a 50% chance to be a Rainbow, and the chance to pull 11 golds would be as big as the chance to pull 11 rainbows? And that would be about 0.05% chance, to give an idea of how your chances to get either the worst or a perfect roll.
Heck, I'd probably buy it! It's been a while since I had to do this kind of mathematics, but I figure it'd already be hard to get less than 3 of either rarity.