It's not, I would like to say that rarity should be the main influence behind a pet, but it isn't, demand is. And every server I go into, like at least half use values of a pet that are close to value lists. It's not value lists people follow, it's the current situation in bgs. Sentimentality does come in, but rarely does unless its like a secret someone hatched, which I have barely ever seen. Also, many people don't like new pets and think older pets are much better, which also influences the economy. Value lists may say new pets are worth this and that, but no one wants them, so you'll have to overpay alot if you're trading them for older pets. As I said, most people don't actually follow the value lists.