I think whatever slingblade wants to sell it for is what he'll sell it for, regardless of what we all paid for ours. 500mil is not a lot when you consider the price of blaze cus that still spawn, yeah there hard to get but they'll never compare to a dread warhorse or old sl warhorse that you can't get anymore.. Those two pets don't spawn anymore, now i'm not saying i'd ever pay that much for a dread warhorse, I wouldn't even price them in the same category as an old sl warhorse just because theres obviously quite a few of the dreads in-game. I can say though that I see old sl warhorses about as much as blaze cus if anything I may even see blaze dogs more then old sl warhorses. Hope that helps at all, but it's really up to the seller to determine what he wants for it... It may sell for 200mil today and someone could go and say "Well one just sold for 200mil so thats the going price" and then one tomarrow sells for 500mil. It all depends on what the sellers asking price is. Regardless of how much he's asking for, it is ultimatelly up to him on whether or not to sell it. The ones that want one bad enough will pay the price.
Also you can't price anything based on a couple of years ago, the gold market fluctuates so much in price and whenever gold gets cheaper items become worth more in-game. When gold gets more expensive per mil then items are worth less in-game, its that simple. A couple of years ago gold prices per mil I believe was a lot more then what it is now. Incase i'm wrong and they are the same as what it was a few years ago. It wouldn't matter anyway because as time goes on, people quit playing and new people join the game. The rare ones that are out there can get lost due to game mechanics or due to the player. A good example of this would be recently whenever Atlantic shard went down and some people claimed to have lost there pets. People can lose pets just like they can lose anything else.
Since you can't "dupe" pets in the game, I would say they are more valuable then any item imo. Once the pets gone its gone for good, there's no "oh wait I have 20 more in my chest at home" lol. There are lots of these out there shard wize, but a lot of them are probably on deactivated accounts. New players joining the game and buying accounts don't know when they have a rare and go ahead and sell it for to cheap. Then all someone has to do is say "This one sold here for this cheap so thats the going price" ya know?