Totally agree, Darkmoon Rabbit is hot right now because it hasn't been out long and is purple colored. There will be a steady supply of these as Darkmoon Faire continues to happen every month, and demand will saturate very quickly at the 100k+ price point.
Soul Trader (and all of the rare* TCG pets) have an actual real-world price, set mostly by eBay commerce. For Soul Trader, it's difficult to find under $100. Spectral Kitten is in that ballpark as well. This translates to sell prices of 100-300k gold, depending on server.
When MoP launched and pets suddenly all became BoE, there was a relative glut of TCG pets, at least on my server. I suspect that a lot of people had gotten these pets either randomly in packs or for cheap, or maybe they had simply stopped caring about pets and decided to cash in. In any case, it was a good environment for picking up TCG pets for gold, but those days are already gone, and they gave a false impression of the actual scarcity of the TCG pets. The supply of TCG pets is legitimately limited by the print run of the loot cards (as opposed to Darkmoon Rabbits, which are spontaneously generated on every server multiple times during DMF). Thus you can expect the TCG pet supply to never be as strong as it is now*, whereas Darkmoon Rabbit and any ingame generated pet will simply continue to increase in supply.
* just to be really clear, there are TCG pets that are not at all rare, such as Eye of the Legion, the Scarab, Landro's Lil XT and KT, Grell, Nightsaber Cub, etc. They sell for $5 on eBay routinely. The rare pets are e.g. Soul Trader, Spectral Kitten, Bananas (duplicated functionally by Baby Ape), Rocket Chicken, Hippogryph Hatchling, Dragon Kite.