It turns out that there are now some cardinals being raised by American hobbyists. A number of projects in the Amazon to encourage sustainable fisheries seem to be afoot.
Not only that, but a fair number of cardinal tetras are now entering the market from commercial establishments in Eastern Europe.
This may explain why they aren't dying off after a few minutes in the shop like it seemed that they used to in the 1980s. Just as neons are much easier to breed than they formerly were, maybe cardinals, still way harder to breed than neons, will be less difficult in the future as they go through more captive generations.
This has happened before. In the 1930s a pair of angelfish could cost a couple of months' wages. They were considered almost impossible to breed. Look at them today.
Fancy guppy prices seem breathtakingly high on some web sites and Aquabid. By my standards they are! However there are sometimes ways - legally - to get those fish without a second mortgage. ;)
Fish magazine ads in the 1960s show fancy guppies for sale for $15-20-25 a pair. When one figures in inflation (the minimum wage was about $1/hour then) today's show guppies, bigger, more diverse and even more magnificant than ever, are not quite so prohibitive.