Ursula you are on the right track. They can't be mericloned.
But there is another reason, Cattleya seed pods contains hundreds of thousands of seeds, even millions! But if you make a cross and flask up say,
50,000 seedlings and sell 10,000, the total cost per seedling gets divided by 10,000.
But slippers produce only dozens to a few hundreds of viable seeds. So you take the cost of those flasks and divide it by 25, 50 or even 200, the price goes up from there.
Then we apply Teds' guess, " demand" and the price gets adjusted again.
This is why some crosses the vendors can't even 'give them away'! Everyone has them so nobody wants them. But for the slipper, not everyone has them, they want them to sell but not everyone gets them and the few that they get they sell for mucho dinero!!!
