There have been many waves of duping, the least subtle being the lvl 6 tiger cubs. Actually if you look on UJ for cubs, it's quite comical -- scroll to the bottom and look at the graph showing prices for all realmshttps://theunderminejournal.com/battlep ... pecies=242
As you can see, there's a massive cluster of cubs being sold for exactly 60k across many servers. That's not random. I checked a handful of realms and saw multiple lvl 6s. So either that loophole hasn't been fixed, or people are still selling off cubs from the earlier waves of duping. I have also literally never seen a report here or on Bliz' forums of Bliz taking someone's cub away. The main point here is that many of these lvl 6 cubs were never removed, so the effect on supply is long-lasting.
There is no smoking gun for Soul-Traders that I'm aware of. But the decline in prices has been very similar to cubs over the course of MoP, which is very suggestive of an uncontrolled supply.
Basically, high-end pets are a terrible investment. Bliz has shown no willingness or ability to control illicit supply, which inevitably pushes prices down. There are only two things keeping prices up: 1) lack of awareness among the playerbase (ie legitimate buyers), and 2) the dupers have a cost of doing business (in terms of manhours, banned accounts, etc). Below some price, the dupers will move on to something more profitable. That Spectral Tiger Cub graph is suggestive that this price floor might be around 60k for a caged pet, but that conclusion is a bit of a stretch. If they 'improve' duping methods to reduce the risk of getting banned, or reduce the effort involved, then this price floor could easily drop further.