The price of the Guardian Cub varies wildly by server -- a function of supply and demand. An impromptu Twitter survey suggests that the pet is currently selling for between 6,000 gold and 40,000 gold in game, depending on server size, competition, and a number of other factors. Most realms are currently seeing prices just north of 10,000 gold.
Certainly, the final page of the Guardian Cub saga has yet to be written, and prices will be extraordinarily volatile in the next few days, weeks, and months. Still, even at a conservative exchange rate of $10 for a 10,000-gold pet, players can get a far better (and safer!) deal buying gold through Blizzard via the Guardian Cub than dealing with a gold seller. The difference is stark -- the same amount of gold may cost you $20 or $30 through a third-party site. And even then, you have no guarantee of getting your gold, no guarantee that your account won't be compromised, and no guarantee that your purchase isn't supporting forced labor and account theft.
Will the Guardian Cub kill off third-party gold sales? Probably not, at least on its own. Interest in this new pet simply cannot be sustained long term. But if the last 24 hours of trading on the in-game Auction House are any indication, Blizzard just fired a shot into a multi-billion-dollar gray market.