Well, it's been 24 hours since the Guardian Cub was released, and people have started posting them on the AH and advertising them in trade chat. So far the going rate on my server seems to be 10k to 15k gold. But as was pointed out on WarcraftPets' Facebook, anything above 5k is a rip off. And anything less than 3k is a great deal!
I'd say this is an accurate benchmark. Just yesterday a gold seller was spamming in trade chat that $15 (USD) would get you about 10k gold. So if we take the price of the Guardian Cub ($10), it should bring in at most 7k (and that's if you're not looking to make a profit, just a direct cash to gold conversion).
Anyway, this is just day one of sales. I'm not going to actively follow the price trend but I'm going to make a point to check up on how the cub sales are going from week to week. It's an interesting situation for a companion pet, and one of the first of its kind.