Apart from sentimental value, of course, I hesitate to place any gold amount on them at all. Factor in the requests for years to revert imps to their red colour given the enormous difficulty in spotting green imps in grassy areas such as Hedge Maze and some areas in Ilshenar, and I think the Magincia event red imps have a risky & limited lifespan as a rare pet.
Many of the legacy pets' rarity is only seen on using the animal lore skill to view the stats or resists or dietary requirements. My unicorn looks no different from any spawning in Ilshenar, yet on loring it you can see it prefers meat to fruit. In the same manner that I gave my own two carnivorous unis away years ago, one was given to me in the last six months (thanks Koris!).
On rummaging through one of my older tamer's stable slots, I discovered an old pre-AOS frost spider which deals physical & fire damage, rather than phys & cold damage as they spawn now. I traded that for some Europa gold cloth that I knew to be legitimately cut from sashes.
Rare pets is a funny old market and while I'm certainly no specialist, I personally wouldn't fill a precious stable slot with a red imp even with 8 tamers all with 10 or more slots each. Fortunately, I'm not the only tamer and the OP may well find a buyer for red imps. I'd also have difficulty in selling them knowing full well they may have a limited rarity, however, that, too, may just be me.
Alongside a meat-eating unicorn or ki-rin, or a fire damage dealing frost spider, or a legacy imp with nice resists, a red imp is likely to have limited value to most rare pet collectors.
If forced to put a gold figure on it, I'd go with a hundreds of thousands maximum rather than the million or more mark.